In a private screening room on the outskirts of Toronto, women are screaming at Michael Cera. It’s a sneak preview of the 20-year-old actor’s latest movie, the hipster rom-com Nick and Norah’s Infinite Playlist, and the watching women – in particular, a gaggle of six flush-faced students – are clearly obsessed by Cera, who made his name in hit comedies such as Juno and Superbad, and here, as the titular Nick, spends a screwball New York night searching for love and indie rock’n’roll, while cleverly building an entire performance around three knockout smiles.

On the first one, just before an untimely argument with his co-star Kat Dennings (his would-be paramour, Norah), the girls coo. On the second, when he stares wordlessly at Dennings in a noisy nightclub, they giggle with emergent hysteria. But on the third smile, just before he kisses her, at the movie’s romantic climax, they actually scream out loud, full volume, hands to faces, boyband groupie style. They are announcing, for sure, the arrival of a very unlikely sex symbol.

“That’s a little weird, but those girls probably just saw Juno and went along with that in mind,” Cera says the following day. “You know, people have being doing that for a long time,” he continues, trying nervously to deflect any analysis of his sex-bomb status. “They fill a void in their lives with movies. But they don’t know me personally, in real life.”

However, in person the Cera mystique is overwhelming. In neat blue jeans and a slightly ill-fitting green polo shirt, and with hair hastily pressed forward in a way that suggests he tried, but failed, to style it, he is the epitome of anticool. “I dunno if those girls would be disappointed if they met me,” he says. “It would just be, er, em, different. I dunno.” And yet, and yet. The ever-present hint of a smile playing on his lips plus the devilish twinkle in his hazel eyes suggest a steely inner confidence that, when balanced with apparent outward vulnerability, is fascinating to watch, is very man-child and, ultimately, makes grown women scream.

On his penchant for wearing high-cut shorts on camera, referring, for instance, to the iconic gold running shorts from Juno and, now, to the thin green boxer shorts that he gamely sports in Nick and Norah’s opening scene, he muses: “I think it was just, em, coincidence that both movies have shorts in them.” But then he adds slyly, “But no, I won’t be basing my decision-making process, from now on, on whether there’s a shorts scene in every movie I do.”

The key line here is from Juno, when Ellen Page’s titular heroine stares into Cera’s eyes and gushes: “You’re the coolest person I’ve ever met and you don’t even try!” The response? “I try really hard, actually.” Cera says that the trying really began when he was 12 years old, at Heart Lake Secondary School in the Toronto satellite town of Brampton.

The son of an office technician father from Sicily and a Canadian mother, he decided to abandon the increasingly oppressive peer pressures and fashion trends of teen school cliques and opted instead for a rebellious statement of antistyle: he wore a pink bicycle helmet and comically outdated clothes to class. He says that it was the beginning of something for him, an acknowledgement that if you were prepared to look ridiculous, a lot of life’s worries simply vanished.

“It was very liberating,” he says. “But it was also very scary. We were constantly afraid of being beaten up.” Luckily, Cera, who had been acting intermittently since he joined a drama club at nine years old, was soon rescued by a full-time role in the Canadian TV series I Was a Sixth Grade Alien. He finished high school by correspondence, which, he says, “was a little bit bogus, because you did as much or as little as you wanted”, and then graduated to higher prospects, playing Sam Rockwell’s younger self in Confessions of a Dangerous Mind, and the younger Bluth brother in the cult TV comedy hit Arrested Development. It wasn’t until Juno became a worldwide smash and Oscar champ that everything went interstellar for Cera.

“When you’re in a movie that’s really popular, it’s a strange life change,” he says. “Literally overnight people recognise you on the street. Sometimes they’re nice, sometimes they’re not and sometimes they just lose track of how to treat other human beings.” He says that on the positive side, he now has more work than he can handle – among others, the big budget Harold Ramis comedy Year One (about life in prehistoric times), and the teen drama Youth in Revolt. On the downside, he’ll probably never realise a childhood dream of becoming a rock-star guitarist. “Now that people know me as an actor, I don’t think they’d be able to get past that and listen to the music.”

And yet, even though his life consists of flitting between Brampton and LA, and simply hanging out with friends and girlfriend Charlyne Yi (the stoned scene-stealer in Knocked Up), he’s still hoping for some vague return to anonymity. “It’s like Arrested Development,” he says. “That had a very devoted cult following, which was really nice. And it made me think that I’d rather be a handful of people’s favourite thing than, you know, the next big thing.” Some hope.

Nick and Norah’s Infinite Playlist shows at OWE1, Oct 19, 8.30pm, and at OWE2, Oct 22, 3.30pm. It goes on general release on Jan 30

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