Seth Rogen - Is it a producer? A writer? An actor? No, it's super-Seth!

The fat, funny, geek of Knocked Up has become Hollywood big-budget hero The Green Hornet. Gill Pringle finds out how

Seth Rogen is almost unrecognisable as he confidently tours the back lot of Los Angeles's Sony Studios. Yes, he still looks like Rogen – albeit 40lb lighter – but it's the difference in attitude that makes him seem an altered man.

Serving triple duties as star, writer and executive producer on The Green Hornet, he has much invested in the film's success. He's not just bringing his name to it, he's also betting his reputation on a gut instinct about what makes great comedy – which is one of the most dangerous games you can play in Hollywood.

Still, having begun his comedy career doing stand-up in his native Vancouver at a precocious 13, later landing roles in Judd Apatow's acclaimed TV comedies Freaks and Geeks and Undeclared, Rogen would seem as safe a bet as any. In the 15 short years since the chubby, red-haired, bespectacled schoolboy first took the mike on stage, he's forged a serious comedy track-record that includes memorable roles in hit films The 40 Year Old Virgin, You, Me and Dupree, Knocked Up and Observe and Report. Rogen, 28, often dismisses his humour as the machinations of an unattractive boy eager for female attention, but he's still managed to package those instincts into something millions of cinema-goers are willing to pay for – not to mention the big studio bosses.

The son of non-profit-worker Mark and social worker Sandy quit high school for Hollywood at 16, and has never looked back. Along for the ride was his one-time high-school pal and writing partner Evan Goldberg, who remains Rogen's co-writer on most projects. The pair were recently hired by Fox to rewrite the script for comedy Neighbourhood Watch, while another script for Jay and Seth vs. the Apocalypse, starring Rogen and Jay Baruchel, is currently at the draft stage.

"The sidekick story is what really drew us to The Green Hornet," Rogen says today, explaining how he and Goldberg came up with what they hope is a winning formula. "As nerds, we always wanted to do something about sidekick and hero because we figured 'sidekick' was the most demeaning term in the history of the universe. There's two people out there, risking their lives, getting shot at and one of them's a sidekick?!

"The Green Hornet is kinda the one character where the sidekick is more famous than the hero. I mean, everyone knows about Bruce Lee, who played Kato, but almost no one knows the name of the actor who played The Green Hornet on the TV show. Ever since we were kids – like the dorks we were and still are – we dreamed of making one of these movies, and this seemed the perfect entry."

Having shed so much weight for the role, one might be justified in assuming some kind of tights and cape were involved in Rogen's costume as Britt Reid and his alter ego The Green Hornet. Not so. "I said 'no' all along to the tights and capes," he says, slamming down his fist on the table. "Well, I suppose I would have put the tights on," he says meekly, "but only if there was some justification for it. I'm not averse to that. I've not been overly precious. As long as there's a general green feel to it, I felt good with it."

The Green Hornet was announced more than three years ago. The pair started on it while they were filming Pineapple Express. "You know, your agent calls and says, 'these movies are looking for writers...' Generally, we say, 'nah, we'd rather do our own thing'. But when we heard it was The Green Hornet, we thought it could actually be the one that would let us tell the story we wanted to tell. We're very realistic that the fact that it's called The Green Hornet is the reason the studios are willing to make such a big-budget version, but to us that was kind of incidental in them giving us enough money to do our story properly."

As producer, Rogen found himself in charge of a $120m budget. "It's insane. But it's not so much the specific amount of money that's stressful, it's all the things that go along with making a movie of that size. We have multiple camera units a lot of the time. There's blue screen, there's action sequences that are extremely complicated to film. It's just infinitely more complex than any movie I've worked on before. It's a lot to wrap your head around sometimes."

How did he take one of the lamest comic-book heroes in history and polish him up for big-budget worthiness? "We watched every superhero movie ever and talked about what they had, what people expect from them, and how we can subvert that."

Like any Rogen project, The Green Hornet involves a loose, improvisational style. "It looks like people you know hanging out, and the conversations feel like conversations you and your friends may have actually had at some point or another. Of course, we do write a script, and it has to be a somewhat acceptable script in order to get a studio to put this much money into it. But we treat the script like a worst-case scenario, basically. We film the script and then we try to think of something better as we're filming it. And when you have people like Christoph Waltz, you're crazy not to let them improvise. Christoph has an incredible grasp of his character that I don't, so far be it for me it to stop him saying whatever the hell he wants." Rogen snapped up the Oscar-winning Austrian actor just months before the buzz began on his Inglourious Basterds performance. The cast also includes Tom Wilkinson, Edward Furlong and Cameron Diaz.

Today Rogen's conversation is littered with studio-speak, demonstrating how far he's evolved from the goofy, immature comic persona of his past. He recently became engaged to his girlfriend of more than six years, writer Lauren Miller. "I have a girlfriend who is far prettier than I should have," he jokes. The pair have been together since Rogen got his first break as a staff writer on Sacha Baron Cohen's Da Ali G Show in 2004.

It was Apatow who was most instrumental in his success, casting Rogen in a small role in Anchorman: The Legend of Ron Burgundy, with America's reigning funny guy Will Ferrell. A year later, Rogen featured in Apatow's directorial debut, The 40 Year Old Virgin, and was also given a co-producer credit. Rogen loves to tell stories about under-age drinking in school, his penchant for pot and the geek lifestyle, but one doesn't get as far as he has by being stoned all day.

Green Hornet producer Neal Moritz has nothing but respect for Rogen's work ethic, saying: "I never suggested to Seth that he lose weight – it was his idea and there was a point, actually, where I thought maybe he was too skinny. He had a trainer and ate fixed low-calorie meals. He turned down the Krispy Kremes. It's amazing how much willpower he has.

"Seth is the hardest-working actor I've worked with by far – not only as an actor but as a writer and producer as well. He leads by example. He never leaves the set, which means other actors don't leave the set, which means we get an extra four or five shots every day and, if you multiply that by 70 days, it's an extra 300 shots."

When I first interviewed Rogen six years ago, he said boldly: "I'm proud to say that I've never had a normal job. I started doing stand-up when I was in high school, purely as a measure to never get a proper job. I'd be terrible at anything else." By his own measure, then, Rogen is living the dream – although "producer" does sound dangerously like a real job.

'The Green Hornet' opens on 14 January