Rolling in gross profits

He was the only funny thing in Evan Almighty. He had scene-stealing cameos in The 40-Year-Old Virgin and Knocked Up. But since Superbad (his first leading role) knocked Knocked Up off the No.1 US box office spot with an opening weekend of £16million, Jonah Hill has literally become Hollywood's biggest young star.

A fact that's clearly discombobulated this amiable Californian 23-year-old.

Illegally smoking in his swish hotel room, as if his Marlboro Light is the one rock in an unsteady world, Jonah attempts to flick ash into a half-empty Coke bottle and ends up scattering it all over the table.

'F***! That's the first thing you're gonna write about, isn't it? Every time I do something like that, the article starts off with "bumbling idiot, Hill..."

'I was eating lunch one time with an interviewer and a single crumb fell on my shirt - and he started his interview with "crumb-filled shirt". There was one little crumb!'

With Hill, it's easy to equate actor with character because (a) he looks, in today's faded jeans and favourite XXL De La Soul T-shirt, just like the bumbling, crumb-strewn slackers he portrays; and (b) in Knocked Up, the bumbling, crumb-strewn slacker he plays is actually called Jonah.

'We thought it was such a funny joke to use our real names. Then, as we were shooting, I'd go: "Oh f***, people are going to think this is me."'

Also, Hill's approach to acting is very naturalistic. He's a key member of the post-Ben Stiller 'Frat Pack', a new wave of US mainstream grossout comedy led by Judd Apatow, producer of Superbad and director of Knocked Up and The 40-Year-Old Virgin, who kicked off the movement with hit production Anchorman: The Legend Of Ron Burgundy.

'Judd has always been big on improvisation,' says Hill. 'I've done movies where you're asked not to improvise or write yourself and it makes you feel useless, like you're just hired to say stuff. Comedy shouldn't feel scripted. We're trying to make it feel real. But I think I'm way nicer than most of the characters I play.'

That's not hard, given that, in Superbad, Hill plays Seth, a tactless, selfish, porn-obsessed virgin aspiring to be 'the Iron Chef of pounding vag', which led me to expect our interview to be conducted on a level somewhere between Beavis and Butthead. However, in real life, Hill is mature, super-polite and articulate.

Although privately educated in California, where he was friends with Dustin Hoffman's children Rebecca and Jake (a connection which led to his film debut in I Heart Huckabees), he insists: 'I'm kind of almost embarrassed by the fact that I grew up in Los Angeles.

'There's such a connotation of, oh, you're probably like a Hollywood brat. I mean my dad's an accountant. What I was interested in was telling stories and being part of movies but not the other stuff. I've never even met Lindsay Lohan. I doubt she would know who I was. Being a writer seemed more my thing.'