He rewrote Jim Carrey's early comedy, The Cable Guy. Garry Shandling hired him as writer and producer for The Larry Sanders Show. He met Ben Stiller at an Elvis Costello show and ended up producing The Ben Stiller Show for Fox. He teamed Will Ferrell with Ben Stiller in Anchorman: The Legend Of Ron Burgundy and then with Ali G in Talladega Nights, both of which he produced.

Still only 39, he's now the catalyst for a whole new wave of younger American comedy actors and writers who sharpened their wit on his critically acclaimed TV shows, Freaks And Geeks and Undeclared, in particular Seth Rogen, who he teamed with Steve Carrel in The 40-Year-Old Virgin and has turned into an icon for male ineptitude in Knocked Up.

Rogen's just gone top of the US charts with Superbad, which they wrote together. "It's almost like an episode of Freaks And Geeks we would never have been allowed to make because it's filthy, but it's also a very sweet story about the panic of two guys who hang out with each other their entire lives but are separated when they go to different colleges, " he says.

So how has Apatow managed to remain a secret for so long? "I didn't want to be a secret, " he says.

Until The 40-Year-Old Virgin, he was in the middle of a seemingly terminal losing streak. NBC dropped Freaks and Geeks and Fox, which had already axed The Ben Stiller Show, did the same with Undeclared.

"I was shocked, " he says. "I thought the kids I was working with were the future of comedy.

What's fun about now is that all the same actors are becoming successful. So it's a way for me to pretend the show never got cancelled in the first place. In my head I'm still making Freaks And Geeks.

"Television is so conservative. A small minority of people make all these rules. It seems in America, only violence is OK. I love the fact I'm making these movies that really push the boundaries of what you can say and show and they're being accepted in a huge way across the board. With the internet, everyone is seeing everything anyway. So to pretend it's not happening is ridiculous."

You could almost call Knocked Up a home movie. It features Apatow's wife Leslie Mann (they met on The Cable Guy, and she also starred in Virgin) as the married sister of a beautiful TV news reporter who gets drunk celebrating her new job and wakes up to find herself pregnant by Rogen, a slacker layabout she met in a bar.

Apatow's daughters Iris and Maude play her screen daughters, and her husband is Paul Rudd, an Apatow regular from Anchorman.

"When you're telling a personal story, it works better if you know the people you're directing intimately, because you can get them to show some side of themselves that maybe only you know exists. If I need Leslie to meltdown and curse out a doorman who won't let her into a nightclub because she's too old, I kind of know how to get her there. And I've known Seth since he was 16, so when I'm trying to imagine him taking a woman to a gynaecologist and being with her during the examination, I know what he would do.

"I've never had the courage before to be so confessional. For a lot of my career I was the person who supported other people and helped them execute their ideas.

Only with these few movies have I dared to think any aspect of my own life or personality might be interesting."

He'd wanted to be a stand-up since his grandfather, who'd produced the first Janis Joplin record and recorded Ray Charles when he was just a 19-year-old guitar player . . . "He tried to sign Elvis Presley but couldn't persuade the record company to come up with enough money" . . . used to take him to Long Island comedy clubs as a teenager. "I wanted to be those guys. I got a job as a dishwasher just so I could watch Eddie Murphy do his stuff. At school I created a radio show where I'd interview comedians like Jay Leno or John Candy. They thought it was for a big radio show but our signal hardly got out of the school parking lot."

He studied film at UCLA but dropped out after a year ("I still don't know what lenses do"). Eventually he realised he was a writer not a stand-up when he had to go on after a Jon Stewart act. By 19 he was producing The Ben Stiller Show. Right now he has seven films at various stages of production, including Step Brothers with Will Ferrell, Forgetting Sarah Marshall, The Pineapple Express and a mock music biopic Walk Hard, all involving Apatow regulars. "We all help each other, " he says. "It's what Karl Marx intended. It's communism finally working."

'Superbad' opens on 14 September