The clothes?  The attitude?  The fact that she saves the world every week?  Or is it just the hot, sweaty sex?  Behind the scenes at the coolest show on TV.

Sarah Michelle Gellar is having one of her all-but-nonexistent moments off from filming the final several episodes of Buffy the Vampire Slayer, the eponymous heroine of which she is. She is sitting in a director's chair over by the monitors, dressed, as Buffy, in a white sheer top over a black tank and pants. The show, although it has drifted somewhat from the original Valley-girl tendencies of the title character, consistently dresses its leading ladies in the rockingest clothes on television - courtesy of costume designer Cynthia Bergstrom, who seems to have much the same heightened sense of awareness when it comes to catching small-designer-label trends that Buffy has when it comes to fighting demonic evil. (And, as the Manhattan-raised and hometown-proud Gellar notes, "It's very hard to be a show in L.A. and be trendsetting, because the fashions are in New York, and you're competing with every other show that shoots out here. Not to mention that most actresses are all, give or take, the same size - we're all between five-two and five-five, and between 95 and 125 pounds.")

Gellar, 23, who has played the young woman whose lot in life is to battle monsters since the show debuted as a midseason replacement on the WB in 1997, has been here on the set in Santa Monica since early in the morning and will be here until late at night. She has often said that the early assumption of adult responsibilities is something she shares with her character - a former child actress (she was discovered in a restaurant at age four), she has been a subject of public scrutiny at least since her actual high school years, during which she played Erica Kane's illegitimate daughter, Kendall Hart, on All My Children.

Whatever their source, as Buffy and in person, she has a beauty contestant's smile and the hypervigilant manner of a person who doesn't trust anyone who hasn't earned it but who nevertheless needs your vote. She has a physical charisma that in itself borders on a superpower. And at the moment, she also has a very realistic-looking, carefully applied cut across her forehead. She speaks very fast, and her rapid-fire delivery has served her well when negotiating the sentences of Joss Whedon, the show's creator, which tend to be long and to contain many clauses. She is considering the question: What makes Buffy slay? "I think it comes more naturally to her than she'd like to admit," she says. "She says, 'Ooh, I'm always having to do what's right,' and, 'Ooh, it's so hard,' but really, that is her nature. The thing is, with this show, you can identify with so many of the characters. You really take an interest in what's happening to each and every one of them.

"And it's all in Joss' brain. It's amazing to me that one day he had this thought and now he's created this empire, this entire lot. Like, in a couple of days we start shooting the last episode of the season, and no one has any idea what happens. But Joss just keeps saying, 'Don't worry. I have it right here.' "

Joss Whedon has always liked to create imaginary worlds. When he was eleven or twelve, for example, he had one featuring hero Harry Egg, itinerant space traveler, and his androgynous demigod sidekick, Mouseflesh. Ten or thirteen years later, during which interval stuff happened - school, the writing of spec scripts and eventual employment on Roseanne - he had another idea. It was an idea that was more like an image, actually. "It was pretty much the blond girl in the alley in the horror movie who keeps getting killed," he says. "I felt bad for her, but she was always more interesting to me than the other girls. She was fun, she had sex, she was vivacious. But then she would get punished for it. Literally, I just had that image, that scene, in my mind, like the trailer for a movie - what if the girl goes into the dark alley. And the monster follows her. And she destroys him."

And that pretty much is what happens on Buffy. After three years at Sunnydale High School, Buffy Summers has just completed her freshman year at UC Sunnydale. She is a vampire slayer. In every generation there is one slayer whose burden and skill it is to fight evil - primarily, but not exclusively, in the form of vampires. Sunnydale is the center of an extra-heaping helping of evil, because it is situated on a Hell mouth. If the Hell mouth were opened, the world as we know it would come to an end, and demons would rule the earth. Complications have ensued.

In the real world, Whedon, 35, is sitting in his office in a building on the studio lot in Santa Monica where much of Buffy, currently concluding its fourth season, is shot. He is wearing jeans, sneakers, a corduroy jeans jacket and a blue-and-white striped shirt, an ensemble that makes him look sort of like a hip-hop Dennis the Menace. A graduate of Wesleyan who grew up in Manhattan and went to boarding school in England before following in both his grandfather's and father's footsteps as a writer for television, thus forming a direct line of descent from The Donna Reed Show to Buffy, Whedon was not himself a happy adolescent. "I was one of those kids who no one pays attention to, so he makes a lot of noise and is wacky," he says. "But I was funny; I wasn't totally annoying. I decided early on that my function in life was to walk into a group of people, say something funny and leave while they were still laughing. Which is pretty much what I did, only now I get paid for it." (And in the case of Toy Story, which he co-wrote, Academy Award-nominated for it. Other credits include Alien: Resurrection and Speed, as well as the 1992 feature Buffy the Vampire Slayer.)

When he speaks, he tends to look off into the middle distance, as one whose habitual eloquence doesn't make him any less habitually shy. He is also wearing a slightly pained expression, maybe because he still hasn't written the last episode of the season; maybe because, what with Buffy and its spinoff, Angel, he bears the weight of imaginary worlds on his shoulders; but probably because he had an emergency appendectomy earlier in the week.

"Then I wrote my little movie," he continues, playing what appear to be imaginary arpeggios on the arms of his chair, "which was sort of fun. And then they made my little movie, which was sort of less fun but had a very small fun degree. And then this, which wasn't my idea. After the movie, a TV production executive said, 'This is a TV show.' So I thought, 'Well, a TV show needs something that will sustain it, and a California girl fighting vampires, that's not enough. So I thought about high school and the horror movie, and high school as hell and about the things the girl fights as reflections of what you go through in high school. And I thought, 'Well, that's a TV series.'"

But just barely. "You try being on a midseason replacement show on the WB called Buffy the Vampire Slayer and see how much respect you get," says Gellar. Ten or thirteen episodes later, however, during which interval more stuff happened - stuff in the way of character and story development, of a depth and texture that the show's title did not suggest - it was a whole imaginary world. And, outside of The Simpsons, it was the coolest show on television; in fact, it was cool for some of the same reasons as The Simpsons - because it was writer driven; because it was increasingly ensemble driven; and because, at first glance, it was of a genre so fundamentally silly that it could get away with murder. "You can get to the emotional truth of things almost by sleight of hand, while people aren't really looking," says supervising producer Marti Noxon. "It's sort of like, 'Here, look at the shiny vampire,' and behind that, there's something really raw going on."

And, often, there is - for one thin, people's feelings get hurt on Buffy, and when they do, instead of the usual resolution in the final act, it resonates over whole seasons, and beyond.  For another, Buffy is one of the most sexually blunt shows on the air and, for its family-hour time slot, almost subversively so.  You have only to look at the parallel suggested by the imagery of Sunnydale, the fictional town where the show takes place, being situated on a Hellmouth, a portal that has to remain sealed to avoid dire, world-changing results, to see that it's not a show that takes the consequences of sexual activity lightly.  "It's something we deal with," says Whedon.  "Because it's something that's on people's minds.  But on a horror show, if you do something - anything - you are going to be punished for it.  I'm not out to say it's bad.  And I'm not out to say 'Everybody go have sex now.'  The fact is, people do have sex, and sometimes it gets complicated, and that's what we want to get at."

Anyway, the characters, most of whom graduated from high school last season, have sex, and some of them have plenty of it, and that's not even the subversive part.  The subversive part is how integrated the characters' sexuality is - and not just on the obvious, symbolic level, where teenagers and vampires are united in being ruled by forces within them that they can't always control.  What really makes Buffy subversive, especially in its depiction of female sexuality, is that where, say, Ally McBeal wants a boyfriend! or doesn't! or, wait! she does! - and hats off to that show for examining the situation of a single woman who wants! or doesn't want! a boyfriend from every conceivable angle, plus the opposite one - the characters have sex consequences, school with consequences, popularity with consequences, even endlessly repeating replays of Cher's "Believe" with consequences, positive and negative.  (Except the Cher thing.  That was just negative.)

On one of the three soundstages in the lot that Buffy built, the four cast members who have formed the nucleus of the ensemble since the first season - Gellar; Alyson Hannigan, who plays Willow; Nicholas Brendon, who plays Xander Harris; and Anthony Stewart Head, who plays Rupert Giles - are gathered on the set of Giles' apartment, rehearsing a scene in which they are discussing their battle plan for confronting Adam, the demonoid (like an android, only demon rather than human in basis) who has ended up as the ultimate villain in this season's narrative arc.

Head, whom the scene calls upon to move from the couch to the bookshelf and back, flubs a line and improvises a new ending - "As a matter of fact, you are," he says, adding, "Could I suck any more?"  Gellar, her blond hair styled into modified Ray of Light curls, is sitting on the floor whittling a stake.  She wants to know whether the plus-size medicine bag full of weapons at her feet is the bag she will now have to carry through the rest of the episode ("I was hoping for Prada").  Hannigan, the only cast member in the clothes she will wear for the actual scene - a pink-and-white baseball shirt with a kitty cat on it and gray jeans - sits on the couch bouncing a small rubber ball up and down her arms.  Brendon, who is wearing an oversize blue sweater and baggy pants, is having minor trouble with his lines.  Like much Buffy dialogue, they conflate exposition and wisecrack.  He changes a joke about why Buffy should regret having taken Spanish instead of Sumerian to one about why she should regret having taken French instead of Sumerian.  "Spanish," says Gellar as he concludes the speech.  "No, it's French now," he explains, "because you already established that you spoke French."  "Ooh, watches the show!" says Gellar, mock impressed.

"Buffy," which by its very nature involves a lot of stunt work, visual effects, makeup and difficult-to-deliver dialogue involving things few actors have reference points for in their real lives, such as the history of the Feast of St. Vigeous, is an ambitious and complicated show to execute.  The cast - which has developed a policy, not always supported by Whedon, that the first person having a line including something like, say, the Sisterhood of Zhe gets to establish pronunciation - appears as weary as you would expect it to when shooting the twenty-first episode of a twenty-two-show season on a week when the boss, who is not known for delivering scripts at anything much sooner than the second-to-last moment when he hasn't been unexpectedly hospitalized, has had emergency surgery.

Like the sources upon which Whedon draws in creating the imaginary vistas of Sunnydale, the actors' origins are far-flung.  Hannigan, like Gellar a child actor, grew up in Atlanta ("Where I'm from, the biggest deal was, like, 'Hey, we got a national commercial, whoo-hoo!'") and has the sweet, smarter-than-she-thinks-she-is, goofy-sexy charm of her character.  Brendon, who grew up intending to play professional baseball ("But when you throw hard, the tendon that connects your elbow to your shoulder completely stiffens up, and that doesn't happen with acting.  So I have more fun with acting."), had done only a line or two as an extra on sitcoms before landing his role on Buffy.  And Head, who was previously best known in America for his part as the guy in the serial-seduction Taster's Choice commercials of some years back ("I wasn't born to fanciness.  I achieved it through a commercial that paid well"), has a résumé that encompasses everything from stage work in London to, well, Taster's Choice commercials.

After the scene in which the mysterious plan for foiling Adam is devised, Gellar departs to study lines for her next scene, with Marc Blucas, who plays her new boyfriend, Riley Finn, a UC Sunnydale student who is also a demon-fighting commando with an underground paramilitary organization called the Initiative.  (Buffy's previous romantic interest, the soulful, good-guy vampire Angel, played by David Boreanaz, had to move to Los Angeles so that she could have a more normal life and he could have his own show.)  That Blucas, as Riley, has carved out a place for himself in Buffyworld at all is a tribute to his own charm, which has a polished quality similar to Gellar's one-two punch of guardedness and gleam.

It being a beautiful afternoon, Brendon and Hannigan, who are done with their scenes, repair to a picnic table in the mock graveyard, where, along with Head, whose family lives in England and who is trying to conduct some of his personal life via cell phone from his trailer, they have agreed to take part in a Buffy roundtable.
What are your favorite episodes?  I like it when you get to be bad Willow, with the major cleavage.
HANNIGAN: I enjoy it as well.  Nick and I had a lot of fun with that.  I had inserts that are made of...
BRENDON: ...actually, human breasts.  It's part of the donor program.
HANNIGAN: ...saline.  Have you tried those Water Bras?  I don't usually wear a bra, but a friend of mine convinced me to try it, and I wore it for the first time the other day.  I couldn't stop squeezing my boobs.  It was cool.
BRENDON: And they have ones with nipples now, right?
HANNIGAN: You know, what's really great about those things is, when you take them out, they get a little sweaty and you can stick 'em against the wall.
BRENDON: It's like spaghetti.
What do you feel you personally contribute to the show?
BRENDON: I've pretty much taken away.  I've sucked and sucked and sucked.  We interpret.  But then Joss comes down and says, "No, do it this way."  So I think the fact is that we all take direction well, when it counts.
HANNIGAN: I can hit a mark.  I can find lens like nobody's business.
BRENDON: I don't even know what a lens is.  I started at twenty-five.  I'm like, "What's a mark? Light?"  I don't understand that stuff.  Like what was it yesterday - "action"?  What's that supposed to mean?  Yup, I'm ready.  Just say, "Go."
[At this point, Head arrives, looking miserable.]
HANNIGAN: But Nicky was nominated for a Saturn Award.  In fact, everyone in the cast except for Tony and me was nominated for a Saturn Award.
What's a Saturn Award?
BRENDON: They're the sci-fi and fantasy awards.  But let's talk about that.  Why didn't we get any Golden Globes?
HANNIGAN: The Golden Globes is because we served them [the Hollywood Foreign Press Association, who is responsible for this particular honor], like, frozen lasagna.
HEAD: [To Hannigan] You know that SFX voted you and me the second-sexiest people in science fiction.
HANNIGAN: But we didn't get nominated for a Saturn Award.  We're the second-sexiest people who can't act.
HEAD: But we're never going to get a bloody award, because they don't know whether we're a comedy or a drama, and you have to be one or the other.
How about the subtext of the whole thing?  Do you spend much time considering the subtext of the whole thing?
HANNIGAN: No, not really.  We spend most of our lives doing the text.

"For me, the show is about a young woman finding herself," says James Marsters, who plays the punk-rock, currently vampirically impotent Spike, of the subtext.  Others also made this point, but I just wanted to mention Marsters because he is so incredibly good and I don't have room for him anywhere else.  Likewise, a shout-out to the impeccable comic timing of Emma Caulfield as Xander's love interest, the 1,100-year-old former vengeance demon Anya; and the tentative, centered appeal of Amber Benson as Willow's love interest, Tara.  (Willow used to date Oz, who was played by the peerless Seth Green and was a rock musician and a werewolf, but she is now having an affair with a woman, who, like her, is a Wicca practitioner.  Did I mention?)

For all that these matchups are often played for comic effect, they are at least as often played, as Whedon notes, in almost embarrassingly deadly earnest.  Most often, it's both.  (As in the last episode to air before May's season closers, in which the Buffy-Riley love connection unlooses the retributive spirit of past adolescent sexual trauma on a fraternity party, causing Xander to exclaim, "There's ghosts and shaking and people going all Felicity with their hair!  We've gotta go in there!" - which is extremely funny.  But the episode is also a perfectly constructed meditation on the role of sexual desire as it is reflected in the relationships of the show's various couples.  Plus - Giles sings!)

In fact, although rarely does an episode pass when at least one vampire doesn't get dusted, and the show may never win a Golden Globe, when it comes to dealing with what's inside you that you can't control, Buffy is the most realistic show on the air, and Buffy, who spends most of the above-mentioned episode having unusually sweaty sex for prime time, is one of its best role models.  "She's driven by her emotions, and she doesn't always make the right decisions," says Marti Noxon.  "But she totally believes in herself and her own abilities, when it gets right down to it.  And I never had that when I was growing up.  It was, like, Susan Dey.  There's a lot of Buffy's empowerment that's about learning to deal with sexuality - that if you open up to something, it will probably make you stronger, but it's going to hurt.  Other shows that take themselves very seriously in dealing with the real pain of living can put people off, because that's all they're dealing with on the very surface of things.  It can also grow very tired, because, I mean, how many times can a character have leukemia?"

"I definitely think a woman kicking ass is extraordinarily sexy, always," says Whedon.  "If I wasn't compelled on a very base level by that archetype, I wouldn't have created that character.  I mean, yes, I have a feminist agenda, but it's not like I made a chart."

So what makes Buffy slay?  "Basically, a sense of responsibility," says Whedon, "and a need to deal with her excess energy.  I know it sounds cheesy, but that's in her, the way that I have to write.  It's just in her blood."  Maybe sort of like leukemia.  "Basically, high school is all about alienation and horror," says Whedon.  "And I was very unhappy in high school all the time, so it was the great well from which to draw.  I think a few people were happy in high school, and I revile them, although I married one.  And it didn't start getting easier in college, for me anyway, so I knew I wasn't going to run out of pain.  It isn't like, 'Well, high school's over, problems solved.'  People never really get over it or they wouldn't respond to the show the way they do."

"That's the whole point of the show," agrees Gellar later, over the phone, in another of her all-but-nonexistent spare moments, when asked whether Buffy has developed trust issues from the fact that if she trusts the wrong person, the whole world comes to an end.  "When someone breaks your heart, it feels like the world is ending.  And in Buffy's case, that's true.  But everyone feels that.  And that's the whole point."