His big break came because he knew Dustin Hoffman’s kids and he only got the role in ‘Superbad’ after a very close shave – now Jonah Hill is a crucial part of Judd Apatow’s comedy ensemble

IT'S ALWAYS A mistake to expect actors to behave like the characters they portray. Those people who attack soap-opera villains in Tesco always sound deranged to me. But the press junket for Get Him to the Greek– the latest raunchy product from Judd Apatow Industries – conjures up the spirit of the film quite nicely. A quasi-sequel to Forgetting Sarah Marshall, the film finds Jonah Hill, a reserved young record-label intern, escorting Russell Brand, a noisy, dissolute rock star, from London to his comeback gig in Los Angeles. The duo makes a nice comic partnership: tubby Californian neurotic meets stringy Essex motormouth.

Sure enough, while making my way into Hill’s hotel room I can hear, through a thick door and down a long corridor, Brand bellowing, whooping and ranting at the chap from TV3. The entire building is filled with estuary energy.

Meanwhile, Jonah Hill, star of Superbadand Funny People, sits quietly on the sofa like a slacker Buddha. His arms are folded. An electric fan is directed towards his large, unexcited face. There seems no way of avoiding the big dumb question: does the relationship in the film echo that off-screen? Is the real Brand the explosive yang to Hill's placid yin?

“No. It doesn’t really echo it at all,” he says. “He’s actually a really mellow, genuine guy. I feel we have a lot in common. Actually we get along very well. It’s interesting. He has a public persona in a way that I don’t. When I’m on a chat show I don’t expect people to think I’m like the roles I play. But he really does have this persona to live up to.”

Still, Hill has managed to establish some sort of media identity over the past half decade. In 2007 he appeared as a sex-starved, foul-mouthed teenager in the uproarious Superbad. Such was the conviction of his performance that it came as a jarring surprise to learn he was 23 years old. Since then he has appeared as a comedy writer in Funny People, a stoner in Strange Wildernessand a ghost in Walk Hard: The Dewey Cox Story. Yet it still seems hard to think of him as an adult.

“I love this job. I would never complain,” he says. “But it’s equally frustrating and fascinating at times. People meet you for 10 minutes in interviews and then they think they know you.” I am certainly going to pretend as much in this piece.

“Yeah, yeah,” he says, laughing. “When actually it takes months or years to get to know somebody. The truth is that only those really close to me actually know me. I am always going to be different to what people expect when they see me in movies.”

Let the facade begin. Hill was born near San Francisco 26 years ago. He claims to have had a happy middle-class upbringing and was, unlike his character in Superbad, fairly popular at school. We know better than to trust anything we read on Wikipedia, but one line in his entry does rather jump out at you. His father was an accountant.

Boring? No, sir. It seems that Hill's dad was, among other things, the tour accountant for Guns N' Roses. I guess somebody has to explain why replacing the television in the swimming pool comes under "legitimate business expenses". It all sounds a bit like Get Him to the Greek. After all, in the film Hill is constantly mopping up the debauched Brand's disastrous spillages.

“That actually is true about my dad,” he says. “Yeah, it was good to do this film for that reason. I love my dad. He was such a hard worker, and he instilled that in me. This movie is my homage to my dad. The dynamic is parallel: he was a family man on the road with these rock-star personalities. He was on tour with them. He was witness to these . . . ” He searches for the perfect euphemism. “Erm. Eccentric personalities.”

Hill's prime ambition as a child was to become a comedy writer. Nonetheless, at college he began taking acting classes, and he caught a break when he made friends with Dustin Hoffman's children. The great man recognised Hill's talent and secured him a small part in the weird philosophical comedy I Heart Huckabees. Overnight success did not follow. He barely worked in the three years that separated Huckabeesfrom Superbad. When you're a young man, contemplating the future, that feels like a very long time indeed.

There must have been occasions when he regretted abandoning college. Every now and then he must have wished that he’d gone to veterinary college or taken up refrigerator repair.

“I guess so. That period did feel like an eternity. My parents said: ‘We can’t give you money, but you can live at home.’ That helped. But Mom and Dad were a bit scared, I think. Who knows? My thought process was clear, though: I am still very young. Even if I had to go back to college in three years I would still be just 25.”

Judd Apatow saved Hill from life as a wage slave. In the interim he had spent some time staging plays and writing strange comic monologues. But nobody seemed very interested in turning him into a movie star.

"It was odd," he says. "I got a manager after Huckabees, and then I didn't work for ages. Eventually, my manager asked me to write a list of all the people I'd like to work with. So I wrote this list of outrageous people: Martin Scorsese, Paul Thomas Anderson, Spike Jonze, Wes Anderson. And Judd Apatow. My manager was, like: 'How do you even know who he is?' He wasn't the force he is now."

Indeed. At that point Apatow was a highly respected but obscure writer on such TV shows as The Critic, Freaks and Geeksand The Larry Sanders Show. When Hill secured an audition Judd had just produced Anchorman: The Legend of Ron Burgundy, his first big-screen hit, and was busy preparing The 40 Year Old Virgin, his debut as director. Hill secured a small role in that film and, alongside Apatow regulars such as Seth Rogen, Paul Rudd and Danny McBride, went on to become part of the impresario's stock company.

Further Apatow-Hill collaborations include Knocked Up, Walk Hard, Forgetting Sarah Marshalland, now, Get Him to the Greek. The Apatow Gang seems like a tight bunch.

“We are all friends. I see those guys all the time. We go out and have dinner. I just marvel that my friends are so talented. But that doesn’t mean I don’t have other friends who aren’t in the business.”

Hill has just the right sort of profane, demotic eccentricity for Apatow's naughty-boy humour. It remains surprising, however, that the producer elected to cast him for Superbad. Why reach for an adult when creating the role of a 17-year-old tearaway? Michael Cera had already secured the largest part in the ensemble piece, but writers Seth Rogen and Evan Goldberg had become increasingly distressed by their inability to cast the angrier sidekick. One day, while discussing the problem on the set of The 40 Year Old Virgin, Apatow pointed at Hill.

"'Hang on. How young can you look?' he said. 'I don't know – maybe 17,' I said. Judd told me to go into the trailer and shave. He got a camera and shot me in the scene. We were actually in Sony at the time. So he then runs into this building and takes the elevator up and goes into Amy Pascal's office – she's the head of Sony – and shows her the tape. He comes back down and says: 'You're in Superbad.' It was a really strange story."

From such strange beginnings are mighty careers fashioned. As Hill sits sedately before his spinning fan he can, quite reasonably, look forward to a secure future. Pretty-boy leading men come and go, but idiosyncratic character actors – he surely wouldn't object to that description – tend to have better luck at securing professional traction. Later this year he appears opposite John C Reilly in a comic drama called Cyrus. You can still hear his voice in the excellent animated feature How to Train Your Dragon. People like young Mr Hill.

“There is no guarantee of work even now. It’s a very difficult business,” he says. “Like I say, I’m just real lucky to be making movies.” I think he’ll manage just fine.

• Get Him to the Greekis on general release