Buffy The Vampire Slayer Takes on TV By BRIDGET BYRNE BPI LOS ANGELES (March 19) - The creator of ''Buffy The Vampire Slayer,'' hopes that you'll be shaking in your seat - with both fear and laughter. Joss Whedon, who in l992 dreamed up the original feature film about the antics of a Valley Girl slayer, has now adapted the scream and squeal concept into a series whose two-hour premier on Monday, March l0, gave the WB Network its best monday night ratings ever. Whedon's impressive writing credits include some of Hollywood's recent major movie hits, the animated ''Toy Story,'' the action packed ''Speed,'' and the natural disaster thriller ''Twister.'' He has also scripted the upcoming sci-fi sequel ''Aliens 4: The Resurrection.'' The 32 year old writer and producer, who has what could be termed a jolly macabre sense of humor, admits that finding the right comedy/horror balance in ''Buffy'' is tricky, but fun. ''What's fun about this show is, we never know from scene to scene which way it's going to go. A scene that starts out very dramatically could end up quite funny, or something truly horrible could happen in it. So it's not sort of, 'Oh, here's the funny part, here's the scary part.' We really never know what's going to be highlighted.'' However he suggests that the show, which combines elements of the supernatural with the angst of teenage life, will incline more towards horror than humor, noting that ''it owes more to the 'X-Files,' than it does to 'Sabrina.''' Twelve episodes have been shot, starring Sarah Michelle Gellar as Buffy, a typical high-schooler by day, at night a martial arts expert who takes on a legion of evil foes. Because her school happens to be situated over ''a hell mouth,'' her nemises include not only traditional vampires, but all manner of hell-spawn. Consequently this high school is even scarier than most. Whedon says that the reality of high school provides much of the foundation for the show's tension, but ''when we deal with teen stuff it is really more emotional stuff, not so much issue oriented. ''I think the best stuff happens when we remember the sort of human relationships that people have that are really twisted and scary and sort of extend those into horror stories, rather than just have a monster show up. That's where the stuff really disturbs me, when its somebody's parent, or somebody's friend who is turning into something horrible, and it brings up issues that are real and therefore actually very scary. Then there's also death and maiming and all that good stuff. . . and high school, generally,'' teases Whedon. Though he attended school from pre-kindergarten to l2th grade in New York, Whedon actually spent a portion of his teens at a traditional boys boarding school in England, which perhaps brings an extra twist to his view of school days and ways. ''Everything is so turgid when you're in high school, everything is so powerful, so dramatic. I don't think there is a time in life when you really feel that way except in high school,'' Whedon reflects. On the other hand, Gellar, who is now 20, spent most of her teenage years acting. She won an Emmy in l994 for her role as Kendall Hart on the ABC daytime drama ''All My Children,'' and also managed to complete her classes at the High School for the Performing Arts in New York, often refered to as the ''Fame'' school because it was the setting for the musical film and subsequent TV series. ''I think high school scares everyone,'' says Gellar. ''I think that no matter how popular you are, or how unpopular you are, high school is a scary place and we touch on that a lot.'' While a school girl Gellar took time to earn a brown belt in Tae Kwon Do, an credential which is now an obvious asset in playing Buffy, though stunt doubles are used when it really gets to the rough stuff. But not all of Buffy's foes are ugly monsters, she also has a rival in the glamorous school vamp, played by Charisma Carpenter. Neither is she alone in her fight against evil; she has emotional support and expert advice provided by the school librarian, a ''The Watcher'' to her ''Slayer.'' English actor Anthony Head, who is probably best known for the coyly romantic Taster's Choice coffee commercials, plays this role, which Whedon says pays mild homage to traditional British horror heroes like Peter Cushing. Whedon says he has always liked ''monsters,'' ''demons,'' ''things with horns jumping out of closets,'' but he didn't find vampires fascinating until he read Anne Rice's ''Interview With The Vampire.'' The idea for ''Buffy'' came from watching traditional horror movies and seeing ''all the bubble headed blondes going down dark alleys and getting killed. I felt sorry for them, so I thought, 'Why not create a blonde who instead mops up all the evil creatures.''' ''I think people need horror stores. They need the 'big, bad wolf,' and they need something to latch onto, something to project their fears onto,'' says Whedon, who thinks that comedy and horror are more compatible than action and horror, because both are about ''not being in control of your environment.'' APEX-03-19-97 1416EST