He may be barely on serious cinemagoers' radar, but if you have young sons you will know Adam Sandler. The 42-year-old comedian is a covert Hollywood success story, who has raked in millions in a 15-year film career, mostly in puerile comedies, launched in 1995 off the back of a successful stand-up career and a five-year stint on Saturday Night Live.

Films like The Waterboy, Big Daddy, and I Now Pronounce You Chuck and Larry all follow a similar arc, in which Sandler's idiotic child-man learns, usually via large helpings of slapstick and scatological gags, to be a better person. His films are loathed by critics but loved by the studios' favourite demographic group, teenage boys. Sandler's two, largely derided, movies of 2008, Bedtime Stories and You Don't Mess with the Zohan, grossed $200 million each.

He can command up to $25 million a picture but this Brooklyn-born, New Hampshire-raised son of an electrical engineer and a schoolteacher also learned long ago that acting as his own writer or producer is the gift that keeps on giving.

Estimated earnings of £36.6 million last year propelled him to the number-two slot in Forbes magazine's 2009 rich list — some way behind number one, Harrison Ford, but streets ahead of Cruise, Hanks and Pitt.

Sandler has been married for six years to model-actress Jackie Titone and they have two daughters aged three and one. He's Jewish, and relatively observant. He is a generous giver to charity and a fan of wrestling. But beyond these bare facts, and the now-familiar sound of thudding critical brickbats followed by ringing tills, we don't hear much about Hollywood's guilty pleasure.
After his first starring vehicle, Billy Madison, was slammed in 1995, Sandler pretty much stopped talking to the press. Even when he showed a more serious side, and a subtler talent, in projects like Paul Thomas Anderson's Punch-Drunk Love or the under-performing post-9/11 tearjerker Reign Over Me, he's kept a low profile, favouring innocuous chat-show appearances over probing interviews.

Today, though, Sandler is in London to discuss his starring role in Judd Apatow's Funny People. He plays George Simmons, a top stand-up turned naff movie-star who contracts leukaemia and is not improved by the experience, even when reprieved. The bittersweet, rather rambling film is the comic's second full-scale collaboration (after Zohan) with Apatow, another alchemist capable of turning dick jokes into Hollywood gold.

The film draws on a year when Sandler and Apatow shared a flat in LA when they were 21 and 20 and both trying to break into stand-up. It even features video footage Apatow shot of Sandler honing his early act by making prank phone calls. Funny People also stars Apatow stalwarts Seth Rogen, Jonah Hill and Jason Schwartzman as competitive comics in a similar flat-share: Rogen's character, Ira, becomes Simmons's put-upon factotum and gag-writer. What's more, Apatow's wife Leslie Mann plays the woman Simmons loved, cheated on and woos again. Autobiography, anyone?

“It's all made up and all true,” explains Apatow. “It's true to my psyche, but not to events.” Yes, he says, Sandler once threatened to seduce a girl if Apatow didn't make a move on her, as Shwarzman's character does to Rogen's Ira: “But I then dated her for four months, before she crushed my heart.” Yes, Apatow once lapped up spilt orange juice from a counter-top, like Ira, in a misguided attempt to woo a girl away from Sandler. And yes, he hitched his wagon to Sandler, trying to follow his friend to Saturday Night Live. Without success.

Like Ira, Apatow became a gag-writer, then a writer and producer on Ben Stiller's and Gary Shandling's TV shows, before making his film directorial debut with The 40-Year-Old Virgin and following it up with the laddish smash hit Knocked Up. Apatow says he knew Sandler was a “superstar” 20 years ago. He helped to “punch up” the dialogue on Sandler's early movies Happy Gilmour and The Wedding Singer. With Funny People, he was confident about casting Sandler, and about taking risks.

“I wanted to work with him on a project close to my heart, not something where I was for hire,” he says. He was moved to make a comedy about illness because, he says, at his age family and friends are starting to ail. That, and the fact he believes anything — pregnancy, virginity, death — can be both funny ha-ha and funny peculiar. “I still love the sillier, balls-to-the-wall films, where you try to be as funny as possible in 90 minutes, but I really think this is about being honest and truthful.”

In contrast to Apatow's earnest self-deprecation, Sandler introduces himself like a comedian, sticking his baseball-hatted, slack-grinned head round the door and shouting “woo-hoo, hello everyone,” before slamming it shut again. He looks tanned, comfortably middle-aged, and very ordinary. I'm reminded he once addressed a convention of cinema owners in Las Vegas with the words: “My name is Adam Sandler. I'm not particularly talented. I'm not particularly good-looking. And yet I'm a multi-millionaire.'' Right on all counts, ­apparently.

Once settled into a chair, though, he's warm, guardedly friendly, and at pains to distance himself from the misanthropic, sexually predatory Simmons. “My latest theory is, Apatow remembers me best when I was 21 and he was 20,” Sandler says. “He still thinks I'm still the horny young man I was then.” And you're not? “I'm not quite as horny. It depends what my wife is wearing.”

He's also not as angry as Simmons, who claims to have become a comedian to win his remote father's approval. Sandler's family is close. Indeed, he was first encouraged to take the stage of a Boston comedy club, aged 17, by his brother. “I don't come from a tortured place,” he says. “My family liked to be funny and I enjoyed making my father laugh, but only because it was nice seeing him enjoy himself. I wasn't looking for approval. My mother was very encouraging of my career. My dad was … well, quite encouraging. He'd say: Maybe you should be a funny salesman'.”

I tell him Funny People, with its intimations of mortality and frenetic comic routines, looks like an attempt to fuse the two strands of his career, the dumb-ass comedies and the more serious, considered works like Punch-Drunk Love (I don't phrase it quite like that). “That's good, that's true,” he nods. “Judd told me he had four different stories for four different films, then he put them all in this one script. Certainly, there are elements of comedy in it that I am very comfortable with. And there are other parts where it's hard to be on set, going through things.” George Simmons doesn't just suffer the effects of cancer and invasive, experimental medicine, but from a growing awareness of his own loneliness.

Sandler, suddenly mindful that teenage boys might find this heavy-going, adds: “But it's not a conscious effort to broaden my appeal. I'm happy with the movies I've done.” Since Simmons spends a lot of time mawkishly watching his old flicks, it seems reasonable to wonder which of his own movies Sandler would be happiest to revisit. “Definitely Big Daddy, cause Leslie's in that, and it's the first one my wife was in too,” he says, then falters. “Um, otherwise, the ones where I'm skinniest. I'd turn the sound down, tell the kids, See, daddy used to be skinny'.”

Sandler admits that comedy is competitive, and says the most important advice for young comics is to find the stage persona you're comfortable with, like Ira in the film. “Judd saw me go through all of that, trying to see if I was better suited as the quiet guy or the noisy guy”.

There was a measure of competition on the set, too, as he, Rogen, Hill et al improvised parts of the script. “It was a fun set, a loose set, but Judd was very much in control because he was on a mission to make something important to him.”
Were there lots of laughs? “Yeah, especially in the sex scene. Cunnilingus — that's what they call it, right? There was nervous laughter when Judd Apatow said, Please eat out my wife'.” Hmm. It's not too surprising to hear Sandler cite Benny Hill and the foul-mouthed Derek and Clive as early influences. He recently said he expected Funny People to win him an Oscar, which I took as a healthily bad-taste swipe at the Academy Awards' fondness for “sickness” films. Or is he serious? “It's not something I think about,” he says, giving that lazy, ordinary-Joe grin again as he gets up to leave, “but it would be hilarious …”

As I leave Claridges I find myself, through an odd and accidental breach of interview protocol, sharing a lift with Judd Apatow. He seems concerned about how the film will go down over here (it earned a healthy $23.4 million on its first weekend in the States). “How did it play at the screening you saw?” he asks. “Did people like it?” Poor guy, I think. So agonised and anxious and exposed, where Sandler seems so laid-back, able to act the oaf and then step back into comfortable anonymity while the paychecks roll in. Outside on the pavement, a sweat-drenched jogger in sunglasses and a baseball cap ambles past me. It's Sandler. No one in the Bond Street throng recognises him. But then, none of them is a teenage boy.
Funny People is released on Friday.