The geek's revenge: How did Seth Rogen become the hottest talent in Hollywood?

A few years ago, Seth Rogen was a college dropout with a couple of failed TV shows to his name. Now, with yet another box-office hit on the way, the 26-year-old Canadian finds himself the leader of a billion-dollar gang of comedy performers and writers. Where did it all go right, dude?

Seth Rogen isn't going to Germany. The promotional freight train for Pineapple Express, the new stoner action-comedy in which he stars and which he co-wrote, will no longer be pulling in to Berlin. "We were going, but they didn't care for the movie – so we cancelled," he says with the big, gurgling laugh that accompanies roughly every other thing he says. "They showed it to press to get their gauge as to whether people like you would be interested in talking to people like me. And the interest was not there. Hurr! Hurr! Hurr!"

"Germans are pretty funny," he notes. "Well known for their fun and tolerance! Hurr! Hurr!"

He might laugh. Since opening in America earlier this month, Pineapple Express, has held its place in the American box-office top ten against stiff competition – the new Batman, Star Wars and Mummy films, as well as the latest Ben Stiller and Will Ferrell outings. It's a semi-improvised, slapstick drugs caper in which Rogen plays a pot-addled legal official who witnesses a murder and then goes on the run with his dealer ("pineapple express" being a strain of super-strength marijuana). It will probably do just as well when it opens here and all over Europe (except, perhaps, in Germany) next month.

"It's crazy," Rogen says. "The fact that it looks like it'll make $80m – for a $26m weed movie, that's pretty good! The fact that our movie made more than [Stiller's new Hollywood-spoofing war film] Tropic Thunder in its opening weekend, and literally the most famous people in the world are in that movie [Robert Downey Jnr, Jack Black, Tom Cruise], shows, 'Wow, people are kind of open to a new group of guys'."

That new group of guys has been hailed as "the frat pack"; they've also been called the School of Judd Apatow, after the writer/director/mentor whose recent films, including The 40-Year-Old Virgin, Knocked Up, Superbad, Anchorman and Forgetting Sarah Marshall, have grossed $1bn. Up there with Will Ferrell at the head of Apatow's class of loosely-affiliated actors and writers is 26-year-old Canadian Rogen, the face of both a new chapter of cinematic comedy (bawdy humour, big heart) and representative of a new breed of improvisational actor whose Mr Ordinary looks have redrawn the possibilities of who a star can be.

"Before we had to go through a huge amount of red tape and roadblocks to get our movies made," Rogen says. "Now it seems like they'll make anything we write. Which is insane and horrifying really."

It's taking some getting used to. Last month, Rogen was on the cover of American GQ modelling Versace (albeit with a mighty "Jewfro", and squirting a joke lapel-flower). "It's shocking! Once in a while something really weird happens and it's, like, 'What the fuck?' Hurr! Hurr!"

We're having our conversation on the second floor of ' Claridge's hotel, where Pineapple Express's promotional circus has arrived in London after a whistle-stop tour of America. Rogen is installed in one room, co-stars James Franco and Danny McBride are elsewhere. Someone wanders past carrying a prop pineapple. In another room a TV crew are playing back their Rogen interview: "So... what sort of 'munchies' do you like?" asks the interviewer. Rogen, understandably, has had his fill of are-you-a-stoner-in-real-life? questions. "You know what today is?" he booms when we meet. "Today is the second-to-last day I will ever have to talk about Pineapple Express for the rest of my life!" (It's Dublin tomorrow.)

At six feet, he's taller, trimmer and less hairy than he appears on screen. But he's even louder, a great booming presence his writing partner Evan Goldberg knew as "the loud dude" at school in Vancouver, back when his hair used to alternate between bright blue and dreadlocks. Rogen seems to be precisely the boy-man of his films, right down to the hangover he's suffering, the result of a night on the town that finished at Claridge's bar.

"To me all men are boy-men," he bellows. "I don't know any man that's actually mature. Most people I work with are older than me and the main thing I've learnt is that everyone is a dumb as an 18-year-old. They just look like a 50-year-old now. And that's kind of... worse."

Admitting this is part of his appeal, of course.

"We're living the lives of the people we're portraying. The fact that we are young and very in touch with what people this age want gives us an advantage. It feels very honest."

Rogen's movie-stealing turn in The 40-Year-Old Virgin, playing (another) stoner finally gave Apatow enough clout to convince studios his charge could go from lumpy-but-loveable supporting actor to lumpy-but-loveable leading man. The wildly successful result was last year's Knocked Up, in which Rogen played (yes) a stoner facing up to the repercussions of a drunken one-night stand. Knocked Up opened at number two in the US box-office charts; next came Superbad, based on a script Rogen had written with Goldberg at the age of 13 and had been trying to get made ever since, which opened at number one. Knocked Up proved Rogen could open a hit movie; Superbad proved he could write one. With Pineapple Express he's done both.

Comedy seems to need to reinvent itself more than any other movie genre. Mike Myers was succeeded by the Farrelly Brothers; Adam Sandler begat the Ben Stiller/Owen Wilson gang. Does Rogen feel the baton's been passed to him?

"I don't know. I mean, those guys still do great. I'm sure [Stiller's] Night at the Museum has made more money than all of my movies put together. But just the fact that we've been able to sneak our way in there is nice."

Still, he was the producer of Knocked Up and Superbad aged just 24. Does he feel Apatow's cottage industry of misfits has triumphed over the Hollywood machine? "I'm sure a lot of guys think they've won. And they slowly start to make shit. We'll slowly ruin it. Hurr! Hurr! I won't speak now."

It was Rogen's parents who encouraged him towards comedy. Sandy, a social worker, and Mark, employed by non-profit organisations, met on an Israeli kibbutz – "Where I come from," Rogen says, "communism is not a terrible word." They would drive their 13-year-old to Yuk Yuk's, the Canadian comedy chain, watch him do his routine about his bar mitzvah and grandparents, then drive home. "I'm 13," he'd tell hecklers. "In 30 years, I'll be 43. You'll be dead."

He was 16 when Apatow, 14 years his senior and with a writing stint on The Larry Sanders Show and 22 rewrites on the Jim Carrey vehicle The Cable Guy under his belt, cast Rogen in the 1999 high-school TV show Freaks and Geeks. "When I met him he seemed like this really funny Canadian with an odd froggy voice," Apatow tells me. "Under his gruff exterior was a sweet, sensitive guy. He is the exact type of guy I love watching in comedies." Freaks and Geeks, a reaction to Beverly Hills, 90210 – "I hate high school" is practically the pilot episode's first line – minted the Apatow trademark: real, awkward situations shot through with teenage frustration. One minute he was giving you belly laughs, the next he was making your heart lurch in sympathy.

Rogen had moved from Canada to LA to make the show. When both that and Apatow's next project Undeclared were cancelled ("Two of the most acclaimed TV series to ever last only one season," according to USA Today), he started feeling responsible for this kid who'd abandoned college for him. He'd set Rogen and Goldberg writing challenges: come up with 100 ideas for films; turn an idea of his into a movie in a week. Meanwhile Rogen had small parts in Donnie Darko and Dawson's Creek, before Apatow introduced him to Sacha Baron Cohen and he joined Da Ali G Show's writing team, receiving an Emmy nomination.

In the meantime, Rogen and Goldberg never gave up on the stories they'd started writing together, sat in Goldberg's sister's pink bedroom round the family computer. "If we listened when people told us something wasn't possible then we'd have stopped trying to sell Superbad 10 years ago," he says. "We would have stopped trying to sell Pineapple Express six years ago." Now they've worked out that if they ask studios for less than $30m – "that's one minute of Spider-Man" – they'll let them make the movie they want. No interference.

They just handed over Superbad and said "Here you go"?

"Yeah, pretty much!" he roars. 'Hurr! Hurr! It's a good deal."

He says his parents' socialism taught him "a very diplomatic sense of how to operate a team". He and Apatow's films seem authentic partly because rehearsals are often nothing more than the young cast sitting around swapping stories, from which a fair share of dialogue is improvised. They've been known to shoot 1.6-million feet of film – four times more than a typical movie – trying out different punchlines. And they audience-test relentlessly. "We'll switch out one thing and do a whole other test," Rogen says, "just to see how it plays. We really try and make it as good as possible."

His own interest in politics? He raises his left hand and makes a big, fat zero. "I'll vote for whoever is the Democrat. That's all I need to know. Hurr! Hurr!" There was a minor brouhaha over Knocked Up and whether Rogen and Katherine Heigl's characters' decision to keep their unplanned baby might be seen as "pro-life": neither character dwells on the possibility of an abortion. But Rogen says keeping the baby was the only way they were going to get a two-hour movie out of the situation.

Anyway, it's all noise to him. He's content enough in his Hollywood neighbourhood, his apartment's décor half-Moroccan Kasbah, half-comic book shop – somewhere he shares with long-time writer girlfriend Lauren Miller, his turtles and an extensive collection of action figures. His friends Evan Goldberg and Jonah Hill live a few houses away. "We play Grand Theft Auto, hang out in my yard, go to the comic store... Pretty much the most boring, regular stuff you could imagine."

Next we'll see Rogen working with two of his heroes: the director Kevin Smith for his self-explanatory Zack and Miri Make A Porno and playing a stand-up comedian alongside Adam Sandler in Funny People, another Apatow production. After that, there's Green Hornet, an adaptation of the 1930s comic-book he's been writing with Goldberg, due out in 2010, and in which he'll star as the masked crime-fighter.

"I think Seth will have a major impact on the future look of comic book movies," says Apatow. "That's his real passion and I think it will be fascinating to see how his spin on things affects the genre."

And then there's "the greatest thing ever". "Me and Evan [Goldberg] wrote a Simpsons episode!" It is, he says, "the peak" of a CV that includes getting a girl pregnant after a one-night stand, playing a stoner on the run from the law and making a porno.

"Yeah, my parents are pretty psyched about me!" he drawls. "I called my dad the other day and he said 'I cannot wait for you to finish promoting Pineapple Express. If I hear one more story about you smoking weed, I'm going to kill myself!' Hurr! Hurr!"

'Pineapple Express' is out on 12 September