A PARENT’S traditional chastisement springs to mind: it isn’t funny and it isn’t clever. Jackass, which has mutated from a series of gross-out stunts on MTV into a high-grossing Paramount film — the studio that brought you The Godfather, I am reminded — is indefensible. Jackass: The Movie stars a gang of superannuated American teenagers demonstrating what happens when you electrocute your testicles, snort wasabi up your nose in a sushi bar or place a live alligator in your mother’s kitchen (respectively, you feel intense pain, vomit and induce domestic hysteria).

The film features the stunts MTV would not allow the Jackasses to do on air, but given what MTV did allow them to do, the extra leeway is quite narrow. Their TV work was marginally less dangerous but no less stomach-churning. Even typing “vomit omelette”, a vignette from the third series, induces mild nausea in me.

“Oh, those images do not leave. They’re seared into your brain,” says Johnny Knoxville, the show’s primus inter pares. He is dressed in a tracksuit and school tie for our interview in a drop-dead-cool West London hotel. He is almost 32 but could pass for ten years younger, which is useful for such a puerile performer. But he is good-looking and has such an easy, beatific countenance that, if you did not know better, you could imagine he was a great credit to his parents, Mr and Mrs Clapp, of Knoxville, Texas (Johnny can take a lot of punishment but squeamishly gave up on his real name, P. J. Clapp, when he moved to Los Angeles at 18).

Although Jackass is the TV age’s ultimate illiteracy, Johnny, it turns out, reads plenty of books. He is about to start The Thief’s Journal by Genet — “Gene Genet” as he calls him — and has, less surprisingly, just finished for the fourth time The Catcher in the Rye, that anthem to disaffected youth.

This morning he is quite the diplomat. “Calling the Bush Administration jackass is an insult to jackasses,” he says, but he cautions me that he’s not going to say anything else bad about “his own people” while on foreign soil. In any case, it is we foreigners who do not know the meaning of respect. In Sweden a man announced he was going to hit him in the groin, and then did so. In Amsterdam two youths, failing to gain his attention by behaving “wackily” in front of him, came back and threw two margaritas in his face.

“So it didn’t get funny for me until I hit them in the jaw,” he recalls.

Still, we agree, these things are a small price to pay. “For travelling all over the world with my friends doing unspeakable acts and getting paid for it? They sure are.”

Their production schedule is tight, not to say drunken. The Jackasses descend on some sober and conventional town — Tokyo comes under sustained attack in the film — and for two weeks they stay up till 5am drinking and talking and planning. They get up at 9am or 10am, shoot all day, half kill themselves falling off skateboards, shopping trolleys and cascading office chairs and, via the nearest accident and emergency unit, make it back to the bar. The boys love the stunts and they love one another, but the best part of the day is the après stunt.

“I’m married with a kid but the boys do real good. Steve-O has a thing where he sees how long he can go without bathing and still get a girl. And it is a long time, to the point where if he was sitting where you’re sitting, I could smell him. Horrible. And girls still go for him.”

Why do girls like them so much? Do they want to mother them? “I’m sure,” he deadpans, “it has something to do with the television show and the movie. I mean when even Wee Man (the company dwarf) and Preston get laid . . . ”

Jackass, I say, takes celebrity to its ultimate nonsensity? “What’s that?” It parodies a world in which you can get famous by applying leeches to your face and diving into sewage.

“No, that was never our intention,” he says, alarmed lest I am intellectualising. “We started making skateboard videos just to make ourselves laugh, and then, for whatever reason, somehow we got a TV show. It’s inexplicable to us, but there was no motive.

“This has been around since the beginning of time, this type of comedy. You get some caveman falling off a rock and someone’s laughing at them. It’s a real primal thing, watching someone get hurt. It’s funny and it’s accessible. It’s not hard to understand. People of all cultures can watch it and understand what’s going on.”