A few short years ago, cuddly Canadian Seth Rogen was an out-of-work comedy writer immersing himself in the kind of layabout behaviour that would eventually be described as "script research". One Knocked Upand one Superbadlater, and Rogen is the face of the new wave of Hollywood comedy. He talks to Donald Clarke

HAR, har, har! What can you say about Seth Rogen's laugh? A combination of a bellow and a wheeze, it punctuates every second sentence and, with that suspicious near ubiquity, suggests that a lingering nervousness still lurks within his burly, Canadian frame. It is, of course, easy to forget that Rogen is still only 26. When, in 2005, he achieved a degree of fame playing Steve Carell's sidekick in The 40-Year-Old Virgin, he had already been tinkering with comedy routines for close to a decade.

Now firmly established after starring in Knocked Upand writing Superbad- two comic takes on contemporary male uselessness - Rogen is in a position of some power. Yet, sitting in the Merrion Hotel, surrounded by bottles of a popular energy drink, he comes across like an overgrown teenager on a first adventure to the big city.

"I actually was in Dublin once before," he says. "I only had time to go the Guinness brewery. But that was cool, man. I got to have the nicest beer I've ever drunk. Har, har, har!"

Superbadand Knocked Up- both developed with the ubiquitous showbiz hyphenate Judd Apatow - made a great deal of money, but some critics have detected a swaggeringly male, clammily teenage sensibility behind the films. Such sceptics are unlikely to be won over by Pineapple Express. Directed by the incongruously highbrow David Gordon Green, the film stars Rogen as a marijuana- addled layabout fleeing gun-toting villains in contemporary Los Angeles. It is, in short, a stoner comedy.

"Yeah, Evan Goldberg and I wrote this back in 2001," he says. "It took years and years to get it made. We didn't realise that stoner comedies are regarded as a separate genre, like horror films or whatever. We just thought of it as a comedy. The fact that there were drugs in it didn't seem that important. But we couldn't make it until we had all got a bit more successful."

The picture works quite well as a lighter, looser take on post-Tarantino slacker noir. Dishy James Franco turns up as Rogen's dealer and the hilarious Danny R McBride confirms his status as this season's loser-in-chief. But this is still a film in which (tut, tut) illegal narcotics are used with irresponsible abandon. Did the good people at Sony Pictures not object?

"Oh no, not at all. Hey, you can do what you want if you make the film for under $28 million. Har, har, har! We got zero notes. If you are cheap - which we were - they will leave you well alone."

Seth Rogen was born and raised in the distinctly unglamorous suburbs of Vancouver. His parents - both, in his words, "radical Jewish socialists" - sound like rather good eggs: mom was a social worker; dad worked for various non-profit organisations.

"They were funny, I guess. Not so much on purpose though," Rogen laughs.

Anybody who has seen the fine Superbadwill assume they already know a lot about Seth's teenage years. Written by Rogen and Evan Goldberg, his lifelong chum, the picture follows three nervous high-school students as they attempt to purchase alcohol for a wild party. A glance at the poster alone will confirm the film's autobiographical origins. Look at largish, curly-haired Jonah Hill staring desperately towards a dubious future. Does he remind you of anybody?

"We had maybe a few more friends than the guys in the film," he says."We were a little bit more confident around girls, but I'd say it's a pretty good representation of how things were." Really?

But Rogan was, as I understand it, doing stand-up comedy from the age of 13. Surely that made him one of the cool kids.

"Oh no, man. Nobody at high school thinks that comedy is cool. Har, har! Cool guys play basketball and football. Cool guys don't do stand-up comedy. Girls don't want to f**k the guy that does stand-up comedy.

"Maybe guys want to smoke weed with that guy. It's still true now in a way. Do you really think more girls want to f**k me than James Franco? Har, har, har!"

At any rate, Judd Apatow, writer, director, producer, thought the young Rogen was pretty darn cool. When the budding stand-up was just 17, Apatow lured him to Los Angeles to appear in the hip (if never particularly huge) comedy series Freaks and Geeks. Despite having yet to hit 30, Rogen has already experienced some of the peaks and troughs of the Hollywood life. Following the cancellation of Freaks and Geeksin 2000, a period of enforced resting set in.

"Yeah, I guess I dropped out for a few years," he says. "You could say I was researching Pineapple Express. Har, har, har! We were writing that film, and writing Superbad, and not getting cast in anything. Really, between 2001 and 2004, I did shit. Then Evan got me a job writing on Da Ali G Show. If that hadn't happened, I would have considered going back to Canada or getting some kind of job. That would never have worked. I'd have been fired from anything else I did."

He says that he was researching Pineapple Express. The hero is a process server whose entire day hangs around the buying, smoking and discussing pot. So, there were a few months lost to smoky contemplation in the early part of this decade?

"I guess. The early part of the film is pretty much based on my life. Now, I was never chased around by Asian gangsters, but a lot of the rest is drawn from my life then. I never really saw that on screen much. You'd get up, go and buy weed and so on. Har, har!"

He goes on to explain how his background made him an ideal candidate to write for Sacha Baron Cohen on the US version of Da Ali G Show. Rogen and Goldberg, both Canadian, saw themselves as buffers between Cohen's English sensibility and his potential American audience. The show was not an enormous success - it took the Borat film to sell Cohen to the US - but it did well enough to finance Seth's continuing researches into Pineapple Express.

It was, however, The 40-Year-Old Virginthat really launched him into the mainstream. Indeed, that film had a major influence on the way Hollywood carries out its affairs. Written in loose, improvisational style, Apatow's first big hit re-introduced the profane, male slacker as an important figure in big-screen comedy. We will be living with pale copies for years to come.

"I don't know what happened," he says. "You know, we just found our stride in movies. We were just trying to do these things on television, but we had a movie sensibility. I just feel when we made that movie it felt right. This is what we were supposed to be doing. It was R-rated and Judd was writing and directing. We were able to improvise and so forth. Everybody, including Steve Carell, got a real step-up at that stage."

He must have wondered what the secret ingredient was in their formula.

"I don't really know the answer. Other people have made dirty movies with an emotional core. I mean Kevin Smith has been doing that for years. But if I had to say what the difference was, I would say that we just have a looser approach. Nobody's sets are as loose as ours."

Seth Rogen is now a big player in Hollywood. Directors squabble to get his name in their credits. Very shortly, he and Evan Goldberg will start work on an adaptation of the ancient crime series The Green Hornet. Yet, for all the buzz around him, he appears oddly unfazed. Indeed, he seems somewhat lessmature than the wasters he plays on screen.

"Well, actually, I've been super-busy the last year," he counters. "There have barely been four days straight when I've been at home in that time.

But I've got a girlfriend and, when I am in LA, we shop, we cook, we hang out. We just do that normal stuff really." I guess that counts as research.

"Yeah. I never had a good idea sitting at a desk. That's not how we got the idea for Pineapple Express. Har, har, har!"

Oh, you big crazy kid!

• Pineapple express opens next week