SLAY BELLE B Y   M A R G Y   R O C H L I N Sarah Michelle Gellar is in her trailer on the set of the WB's sly splatter series Buffy the Vampire Slayer. Last season, in a burst of Martha Stewart vivacity, she transformed the barren module into a vanilla-scented, photograph-filled living room strewn with Christmas lights. Now only a package of Glitter Silly Putty and several bowls of what Gellar calls "agent-manager-producer flowers" adorn the plain white dressing room. "Give me time," Gellar protests lightly. "It's only the third day of shooting." But the truth is, she's running a little behind schedule in more than just her decorating plans: This morning, Gellar's call was moved up so that the hair and makeup department could get busy spackling the undead. As she hurriedly zips up her high-heel knee boots and adjusts her pink crocheted tank top, she's trying not to let the pressure get to her. While most of America is still at the "Sarah Michelle…who?" stage, this blue-eyed, fast-talking, whirring gyroscope of an ingenue has done much in the past five months to answer that question. During her recent hiatus, she began building her film career George Clooney-style, with two features:First, she relocated to North Carolina to play a small-town beauty queen in the suspense thriller "I Know What You Did Last Summer" (due in theaters in October). Next, she jetted to Atlanta to appear as one of Neve Campbell's college friends in Wes Craven's sequel to his horror flick "Scream" (due out in December). Kevin Williamson, who wrote the scripts for both films, reports that Gellar is "great in crazy situations. You know that when you hire her to do a job she's not going to be in the trailer, complaining about everything. She's going to be right out there at three in the morning, barefoot, in the freezing cold, giving you the 10th take." By all rights, Gellar should be sleeping until noon for the next couple of months. Instead, she has emerged from the experience revitalized. "Everyone [on the Buffy set] agrees that I've won the Most Changed Award," says Gellar. "I'm calmer in some weird way. I've learned that I can juggle things." For all her outward calm, Gellar -- not to mention her TV alter ego -- is currently dealing with some emotionally complicated storylines. Buffy's issues involve death, namely her own. In last season's finale, she was killed, then amazingly resurrected. In this season's debut, Gellar says "we will find Buffy is not quite herself. She's got a lot to deal with. I mean, she died." Gellar's real-life concerns involve a different kind of expiration -- the showbiz kind. "Lately I think I've been feeling a little intimidated by everything," she says, wrinkling her brow. "This is what I've waited for my whole life, and now it's happening. I worry that I'm going to do something to jinx it, that something will go wrong. There's this little voice inside of me that keeps going, 'Uh-oh. Things are going too well.' It makes me kind of nervous." Though Buffy has stubbornly refused to bubble upward in the ratings -- the WB's slender lineup just can't compete yet with the big four networks -- television critics have fallen over themselves to praise Gellar's easy physicality and adept comic way with Buffy's wry, pop- culture-soaked dialogue. The series, based on the 1992 film of the same name, positions the 20-year-old Gellar as a pert 16-year-old high-schooler who discovers that destiny has tapped her to keep the streets of Sunnydale, California, free of all things monsterish. In any given episode, Gellar, a brown belt in Tae Kwon Do, can be found flinging herself from balconies, hurling pointy weapons, and slamming grainy-skinned vampires down into the sinkholes from which they crawled. The role of Buffy, played on the big screen by Kristy Swanson, has been enriched for television by Joss Whedon, who wrote the film and is the executive producer of the TV series. Part of Whedon's plan when he began scripting "Buffy" a decade ago was to create a supernatural heroine whose terrifying encounters were reflective of the anxieties all adolescents experience. What makes Gellar's Buffy different from Swanson's is what lurks behind her fearless stance:As dozens of moods migrate across Gellar's face, you realize how lonely her special status makes her feel and appreciate the high emotional price that's exacted when one so young is shown so definitively that the world isn't a particularly safe place. "What Sarah brings to the part is her intelligence," says Whedon. "At the same time, she's got that hormonal, idiosyncratic goofiness that makes Buffy not just the Terminator." In the early stages of the show, both Gellar and Whedon concluded Buffy was a virgin. "Eventually we'll bring up that issue," says Whedon. "The show deals with sex in the sense that high school kids think about it a lot. But it's not like Beverly Hills, 90210. I've often said, 'There will never be a 'Very Special Episode' of Buffy." As with most good series, Buffy has its own distinct personality. Part of the slightly off-center feel comes from the day players who hover around the fringes of the set, their eyes gleaming beadily from beneath thick stratas of vampire greasepaint. Because Buffy has a relatively low budget for a special-effects-laden drama, there's a sense that folks here are accustomed to doing without (for instance, coffee is available backstage, but, sadly, no cups). But a warm community spirit emanates from the cast and crew who are constantly hugging, doling out back rubs, and joking with each other. For Gellar, who ended the previous day's 15-hour shift by bloodying her knuckles during a scene that required some vigorous punching, the cuddlefest is what helps keep her going. As she perches on a table in the set's library and tries to ease her bruised hands open, she blurts out, "You know what? I missed everybody here. Like, terribly." Indeed, what the camera picks up when Gellar stands beside her costars Alyson Hannigan and Nicholas Brendon, who are so winning as Buffy's loyal sidekicks, or Anthony Stewart Head, her starchy mentor, is an atmosphere of genuine mutual support. Gellar has been hanging around sets since age 4, when she was discovered in New York City by an agent. Within months, she landed a string of Burger King commercials in which she chided McDonald's for stingy patties. The campaign won Gellar a place in history as one of the first on-air spokespersons to disparage a competitor -- and McDonald's slapped her employers with a now-legendary lawsuit dubbed "The Battle of the Burgers." (The case was settled out of court in 1982.) After that, Gellar's fierce work ethic took hold:Cast in commercials and TV- movies, she would often be absent from school for months at a time. "Kids were hard on me," says Gellar, who felt so exiled in the hallways that she had no trouble relating to Buffy's sense of deep isolation. "I was always excluded from everything because I was different. That's difficult when you're a child." Gellar was just 15 when daytime television fans saw her step into her first adult role, playing Kendall Hart, Erica Kane's screw-loose daughter on ABC's All My Children. At the time, Gellar, too young to legally drive a car, relished all that came with portraying an evil, lying seductress in her 20s. "It was amazing, playing a psycholoony," says Gellar. "I got to attempt suicide. I shot at people. It was great." For all the job's many perks, though, Gellar couldn't have been pleased with the tabloid tales about the animosity displayed toward her by her TV mom, Susan Lucci. Were the stories true? "It wasn't an easy time in my life," says Gellar. "We didn't have a perfect working relationship. We, um, weren't going out to lunch." (Lucci was unavailable for comment.) The resolution to Gellar's conflict could have been scripted by a veteran soap writer: First came the victorious moment when, during her second year of playing Kendall, she won a daytime Emmy for Outstanding Younger Actress. (Lucci, nominated 17times, has never won one.) Then came the theatrical twist:24 hours after the win, it was announced that Gellar was leaving the show. Gellar says her bosses at the soap had been informed of her plans six months earlier. Unfortunately, the timing of the announcement "didn't [make me] look good. Like, 'Oh, look. She won an Emmy, and now she's gone.' " As showbiz kids grow, they generally turn into one of two types of adults. There are those who've been coddled too long to know how to pay an electric bill. Then there are those, like Gellar, who take self-sufficiency to a whole new plane. She has no free time, no assistant, no maid, yet her Los Angeles home is spotless. And when it comes to her friends, Gellar is the all-purpose helpmate. "She's like the perfect mother in a teenage body," says Hannigan, who woke up from her recent tonsil surgery to find Gellar sitting by her bed. It's anyone's guess where Gellar inherited her nesting instinct. Ask her about her family, and all she'll allow is that she's an only child raised on New York's Upper East Side by her single mother, Rosellen Gellar, a former teacher who now lives in L.A. (Her parents divorced when she was very young.) "Ikeep my private life private," she says. ("Do you get a sense that she has a shell around her?" asks writer Williamson. "It's a thin shell, as opposed to a guarded wall that won't go down. I think it's a protective device.") Apparently, word of her desire for privacy has yet to hit the World Wide Web. There, every factoid and photograph of Gellar has been chronologized in over 40 sites -- an astonishing number, given Gellar's new-kid status in prime time. While it might make Gellar's stomach churn to know that her public can download snapshots of her ex-boyfriends (she has no current romantic interest), she's capable of grasping the peculiar compliment she's being paid -- even when you inform her that in cyberspace, strangers are now ogling fake nudes of her. "I know it upsets some people, but what can you do?" she reasons. "I just hope they're giving me a good body." In fact, in that busy brain of hers, Gellar is already digitally cutting-and-pasting herself to cyber-perfect proportions. "Pamela Anderson Lee," she whoops, "here I come!"