FALL SNEAKS : Bittersweet Thereafter : Fame’s been good to Sarah Polley, but it’s a fight to avoid being ground up in the star-maker machinery.

In a candid moment, actress Sarah Polley, last year’s It Girl at the Sundance Film Festival, admits to being what she calls an inverted snob. By this she means that she thinks acting is a useless profession and actors take themselves way too seriously. But then there’s a point at which she realizes that she’s taking too seriously the whole issue of actors taking the business too seriously.

“Why is what you do any more important than someone driving a bus?” she says earnestly. “I tend to think of this as so shallow and stupid. Then I go the other way. You can choose to make it the most shallow thing in the world, but it doesn’t have to be.”

Polley is really hung up about Hollywood and the star system. Much against her will, she appeared on the cover of Vanity Fair’s annual April issue of hot young actors, which she earned with her breakthrough role as a paraplegic in “The Sweet Hereafter,” her sullen checkout clerk in “Go” and her Pygmalion in “Guinevere,” which is opening this fall. Her ambivalent attitude toward this success made her an almost irresistible target for her colleagues. After all, most actresses would love to have her problems.

“She’s got a really interesting existential dilemma,” says “Go” director Doug Liman. “She’s basically becoming the person she hates. During the shooting of ‘Go,’ I would always joke with her, just to get her angry, about the fact that I was sure that in the next year I would be picking up a Vanity Fair and she’d be on the cover. Thinking that would be a movie down the road, not knowing that it was going to be our movie. It also put me in the uncomfortable position of Columbia publicity asking me to convince her to do Vanity Fair. I was like, ‘Oh, my God, that’s the exact magazine that I would chide her about and say that there can be no bigger. . . .’ ”

Sellout? Polley’s attitude might seem precious, especially given the business she’s in, but she does have an acute understanding of what’s really going on.

“I don’t mind if someone wants to photograph me because they’ve written a piece, but ultimately no one wants to photograph me,” the 20-year-old says. “They want to photograph something they helped to create, so they have to manufacture some kind of image for you, and you either consent or you don’t. There are clothes to be sold, and you’re there to be sold, and there’s a movie to be sold, and you end up being part of an advertising machine. I’d like to think that whatever I do with my life is really important, whether it’s acting or something political.”

As if to illustrate the point, Talk magazine used Polley’s image from “Guinevere” in its mailings to help lure subscribers. A major designer wanted her to model its clothes (she declined). What is it that the studios and the magazine editors and the advertisers are trying to exploit? Polley on-screen has a quiet, expressive, ethereal quality that pulls audiences in. She leaves the viewer wanting more rather than less. (Although she can, if the occasion calls for it, be hard as nails too.) In person she’s a lot more easygoing and girlish than her performances and pronouncements would lead one to think.

“It’s very rewarding to shoot a tight shot of Sarah Polley’s face,” says Audrey Wells, her director on “Guinevere.” “Sarah makes the unspoken moment the most valuable moment of any scene.”

As Polley admits, the fact that she’s resisting all of these advances has made her even more desirable. This, of course, is one way to court publicity without actually appearing to, but Polley has put her career and physical safety where her mouth is. She’s been an activist since she was a teen, having campaigned for the Ontario Coalition Against Poverty, a militant organization that protested cuts in welfare spending. She canvassed, helped organize rallies and attended protests; at one demonstration, a policeman knocked out a couple of her teeth. On another memorable occasion, she made waves with Disney when she refused to remove a peace symbol she was wearing while doing press for the studio.

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Polley’s personal and professional life explains little, other than that she grew up fast. Her mother died of cancer when Polley was 11. At 14, she left home (though she has two brothers and two sisters, on whom she still depends). Before she abandoned herself to political activism, she was a fixture on the Canadian airwaves. She had a small part in “The Adventures of Baron Munchausen,” which she says soured her on big-budget Hollywood filmmaking, and then settled into a long-running Canadian TV series called “The Road to Avonlea.” It was while she was doing “Avonlea” that she appeared in Atom Egoyan’s “Exotica.”

“Sarah has had different stages in her career,” Egoyan says. “With ‘Avonlea’ she became Canada’s Sweetheart. She was adored as this very romantic, almost Victorian child in this wildly loved series. When I first cast her in ‘Exotica,’ I have to confess it was a little bit of a trick. I just thought it would be really interesting to have this young woman who everyone perceived in a certain way play a character who you originally might think is a street prostitute.”

Egoyan got more than he bargained for; the casting stunt became the emotional center of the movie. When he set about adapting Russell Banks’ “The Sweet Hereafter,” he crafted the pivotal role of Nicole around Polley, and he was rewarded with a hushed, completely realized performance that gave a lyrical but often bleak film a much needed glimmer of hope. The film ended up challenging her as an actress and reviving her interest in movies.

“I think when we were making ‘Sweet Hereafter’ she was quite convinced it could be her last movie,” Egoyan says. “To be honest, she’d been coasting. I think she was being herself to an extent, and I think being surrounded by really great actors, seeing what they do and actually having to play a character who was not immediately accessible, where she had to search and work, she found really challenging. And she’s someone who responds to a challenge. She understood the art of acting and the seriousness and the sense of vocation that accompanies that position. And that’s what Sarah needs. Through her passionate defense of the defenseless, that’s become a vocation.”

As she demonstrates throughout “Sweet Hereafter,” Polley is the queen of minimalism--ideal for the camera but less so for the stage, where actors must project. She views this as a limitation and tries to work within it. But perhaps she is selling herself short. In “Guinevere” she plays a sheltered, upper-class young woman who is seduced by a much older man (played by Stephen Rea). Their relationship is symbiotic. He gives her much needed support and approval. She bolsters his badly frayed self-esteem. The role requires her to act her age.

“She had to allow herself to be really goofy and undignified on screen,” Wells says. “And she has a lot of dignity, so she had to strip down for that. She told me that the hardest scenes for her were the ones where she was required to convey a girlish giddiness or happiness. The seduction scene where she starts laughing, she was laughing so hard I thought she was going to pee in her pants. She completely lost it. She doesn’t fabricate anything.”

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Other filmmakers have taken notice. Polley was offered a role in Cameron Crowe’s as yet untitled DreamWorks film set in the early ‘70s rock world. She was excited about the project, but scheduling conflicts forced her to drop out. (Now she may appear in a film with Sean Penn called “The Weight of Water,” though nothing has been made final.) Meanwhile, over the past few months she’s directed a couple of short films, one of which will appear at the Toronto Film Festival, which started Thursday. She views these efforts with a mixture of pride and embarrassment, much as she sees her sometimes strident political activism. It turns out that some of her ambivalence--about acting, about politics, about the media--has to do with being, well, Canadian. Perversely, it’s what keeps her in Toronto.

“If a movie is really great, this is the ultimate Canadian compliment: That did not suck too badly,” Polley says, laughing. “That is as effusive as you’re going to get. I react badly when people are effusive about things. But I think that’s because I’m not used to it. I don’t trust people saying how great everything is all the time.”

She’ll just have to get used to it.