Just another slacker in black leather jacket, shredded jeans, and a couple days' stubble, Ben Stiller strides to the mike in a Philadelphia theater packed with college kids who've just previewed his first film, Reality Bites. While the movie unspooled, the audience seemed sold on the love-triangle comedy, which may well mark a turning point for its stars-Stiller, Winona Ryder, and Ethan Hawke. But now they hit Stiller with questions characteristic of the most suspicious crop of Americans ever raised. ''Why did you cast your fiancee (Jeanne Tripplehorn of The Firm) in that bit part?'' ''Well,'' says Stiller, ''the part had to be filled.'' ''What can you tell me about product placement in this film?'' ''I think it's much more distracting to have a character sip a can labeled 'Soda' than one with a brand name.'' Shyly, one student asks, ''Uh, what was it like to kiss Winona Ryder?'' ''They all ask that one,'' Stiller says after his interrogation, running a hand through his Edward Scissorhands coiffure. He's on the ninth city of his Reality Bites promo trek, doing a half dozen interviews and one screening per day on four hours' sleep. ''I just say, 'It's nice.''' Plainly, Generation X is attracted by Hollywood's kiss-kiss-bang-bang come-on, but the real issue is trust. Reality Bites is made by and for people utterly saturated with film, TV, and music. They are infinitely cynical about any attempt to target the youth market, correctly detecting the greedy media's slavering hunger to gobble up their tasty demographic slice. ''Irony is the only defense this generation has against the commodification of their culture,'' says Stiller. ''It's the most fragile audience-publicity about Generation X stuff will scare them off.'' The showbiz child of comics Jerry Stiller and Anne Meara, Stiller is wise beyond his 28 years about the industry's pitfalls. In his first critical hit, last year's Emmy-winning Fox series The Ben Stiller Show, he played a slimy Hollywood agent in a recurring sketch as only a lifelong Hollywood insider could. He could have parlayed his Emmy into a big-bucks career directing cynical pap, but he turned down Problem Child 2. Instead, he's wooing a cynicism-armored young audience with an utterly heartfelt film-and daring to play an ambitious Hollywood type as a nice guy, not an oily bastard. Yet, inevitably, he is marketing a commodity-and he's upfront about it. More crucial than the products placed in Reality Bites are the youth magnets cast in the film. Until the ultrabankable Ryder signed on, Reality Bites was rejected by every studio on the grounds that Cameron Crowe's Singles grossed $18.4 million-not enough. Stiller further stocked the pond with bit players: Evan Dando, Duff of MTV, Dave Pirner of Soul Asylum (Ryder's offscreen squeeze). The film's MTV video, ''Spin the Bottle,'' is by the sexy pop skyrocket du jour, Juliana Hatfield. There are intimations of integrity in the hawking of Reality Bites. Costar Janeane Garofalo-the film's breakout talent-acted out a Bites scene on Late Night With Conan O'Brien rather than use clips from the film. ''Isn't that a great idea?'' says Stiller. True, he did plant cast members of the Gen X documentary series The Real World in Reality Bites, and MTV sent a crew to shoot its people being directed by Stiller. ''Some people said, 'Oh, you're trying to make money satirizing The Real World (Ryder's character makes a film about her roommates that gets sold to an MTV-like channel).' But it's hard to satirize The Real World-it's very close to over the top anyway.'' Besides, most of Bites' characters are based on 23-year-old screenwriter Helen Childress' actual roommates. Stiller's kissing-scene costar, Ryder, thinks Reality Bites is the antidote to movies kids hate. ''Most movies, like all of John Hughes', are so patronizing about first love,'' says Ryder, ''but it's a powerful, deep, confusing, horrible, great feeling.'' After splitting with Johnny Depp and falling for Dave Pirner, she wildly identified with her character's emotional merry-go-round. ''One day she was in love with him, and the next day she's in love with him, and she's completely feeling inconsistent-she's trying to find out who she is, and I am too.'' But Stiller had to forgo strict authenticity to get the movie made. ''We contractually had to have a PG-13,'' he says, ''so I had to cut a shot where Hawke is taking a long toke off that beer-can bong. Now you never actually see anybody inhaling. I also had to cut Ethan telling me, 'Ah, f -- - you and buy me dinner,' 'cause we couldn't have both a f -- - and a prick. That crossed the line!'' Stiller made a similar cut in his Bites-promoting ''Spin the Bottle'' video: MTV dropped it for a crucial two weeks prior to the film's release because Hatfield, Doris Day's successor as most renowned pop-culture virgin, mouths (though does not utter) the word f -- - in the lyrics. ''It's just ridiculous!'' says Hatfield. Adds a ticked-off MTV employee, ''Snoop gets to parade naked booty in his video, but MTV dumps Juliana.'' Nonsense, says another MTV source. ''In a rush to get it on the air, we overlooked the F- word.'' Stiller quickly reedited, and now it's back on MTV. Stiller knows how to deal the big shots in, but in filming ''Spin the Bottle'' he made them feel left out. He used a four-wall set for the video that permitted 360-degree shots-and prevented record-company honchos from standing around watching. Safe from prying eyes, Stiller's kids improvised with abandon. ''When something happened that was amusing or fun or, you know, sexy, he was willing to let us go for it,'' says Sarah Hayes, 23, a friend of Hawke's who acted in the video. But midway through the shoot, the kids who never, ever call themselves Generation X looked up to see the moneymen looking down on them like biblical elders ogling Susanna. ''They were so frustrated,'' says Hayes, ''that they got one of those scissor-lift things and rose above the set to look in!'' Reality Bites did respectable business its first weekend, but the chief obstacle to its big-time success might be its audience's hostility toward success itself. As Childress puts it, ''I don't think this generation wants a voice.'' Was it hard, a woman at the preview demanded to know, ''working with a theme about, like, reality and kids that people can relate to, and yet to be working on, you know-a commercial movie?'' Stiller replied, a little defensively, ''I personally don't think a movie everybody's gonna see necessarily has to be bad.'' Later, he adds, ''I would've done this movie for nothing-it was close enough; I did it for scale.'' In Hollywood, that's the ultimate test of credibility.