Vincent Cassel: Spaced cowboy

The French pin-up and father-to-be Vincent Cassel is sipping mineral water in Paris's supercool Hotel Costes and talking about how director Jan Kounen introduced him to psychotropic drugs.

"I went with Jan to Peru and took ayahuasca with the Shipibo-conibo tribe. Did I hallucinate? You bet! I saw snakes. You know there are scientists researching into what ayahuasca does to your metabolism," says Cassel. "Some of them think that these intertwined snakes are a visualisation of DNA. Meaning that these Indian tribes are in contact with the meaning of life."

Blimey. For actorly preparation, it certainly beats Renée's doughnut diet for Bridget Jones's Diary. The two men were in the Amazon to research a metaphysical Western Kounen was planning, loosely based on a cult French cartoon.

"I was scared, of course," he says. "It's not like going out and taking ecstasy. It's not recreational. In fact it's the opposite. You can have the worst night of your life." Indeed, when Kounen had returned from his first sojourn with the Shipibo-conibo, Cassel had been a bit worried. "Jan came back saying, 'You know what? Making movies is not that important. There are many more important things in life.'"

Not the kind of insight you'd normally share with your potential star, but Cassel was intrigued. "I thought it was a dangerous game he was playing but I also thought he was brave to seek out a new perspective. Jan said to me, 'Come with me into the forest. You'll see. It's incredible,' and I said, 'Hooyaidonknow'" - Cassel gives a full-body shrug - " 'I 'ave things to do.'"

A few weeks later he was in the rainforest looking at his pineal gland through his third eye. Or something. He finds the experience hard to describe. In fact, after weeks of struggling to find the right metaphor for the French press, he decided to do it again, just to remind himself.

"Jan and I talked about it so much, I began to wonder if it was really what we described. So I went back and saw the shaman. I had another experience. While I was upside-down I laughed so much, because actually we weren't wrong. It's what's in the movie plus 500 different sensations that go through your body that you cannot put on screen."

Apart from the film's trippy climax, were there any lasting effects from the drugs? "There's no toxicity," explains Cassel, "and your serotonin levels, which are very low after having coffee or cigarettes, or any kind of stimulant, actually go up after taking ayahuasca... so it's really something."

So is Kounen's finished film. Although Blueberry begins, conventionally enough, with a shootout in a frontier town brothel, I can't think of another Western in which the good and bad guys duke it out by lying down in a cave and tripping off their heads. Or of another Cajun cowboy for that matter.

Eddie Izzard, who plays an evil, bearded, Prussian prospector, calls Blueberry a "baguetti" Western. French critics have called it pretentious. Not that Cassel will be overly bothered. He's made it his business to rebel against mainstream "bourgeois cinema". He made his debut as an angry young skinhead in 1995's La Haine, going on to star in Kounen's graphic Dobermann and the controversial rape drama Irréversible, co-starring his wife Monica Belluci.

Through it all, he's turned down a steady stream of "baddy" roles from Hollywood. "I realised that I needed to be anchored," he says. "I made movies that are real.They are not just part of the industry."

Cassel will never live in LA. He loves the anonymity of his home town too much. "Paris is great because French people, as you know, are a bit snobby, so even if they do recognise you, they don't like to admit it."

He and Belluci live in a working-class neighbourhood. "I think people are happy that we live there because they are like, 'We have the prince and princess,' you know? But I'm still able to go down to the street corner and have a smoke."

Despite the grey hair growing up from his widow's peak, Cassel burns with a fast-talking, wide-ranging enthusiasm."I think you can really ruin yourself if you start working for the wrong reasons," he says, "such as money. Doing things that you don't like is like sleeping with people you don't want to sleep with. After a point you lose your self-respect."

Despite art-house hits such as Read My Lips and Brotherhood of the Wolf, the versatile and talented Cassel is still relatively unknown outside France. Still, he shows no sign of wanting to go mainstream. "It's not that I'm looking for the most bizarre roles but I like things that are edgy."

So what about comedy? Can this subversive force in French cinema drag the country's lame slapstick into the 21st-century? I put to him the great film conundrum posed by The Simpsons creator Matt Groening: "The French are funny. Comedies are funny. So why are no French comedies funny?"

"I can't talk about humour with a British person because you are the kings!" he says. "Actually, I did do a comedy: Guesthouse Paradiso with Rik Mayall and Ade Edmonson. I had a lot of fun. The balls in the nutcracker. All that kind of stuff." Touché.

Of course, with the release of Ocean's 12 Cassel's low profile will be a thing of the past. He's shooting it at the moment with the director Stephen Soderbergh, George Clooney and the rest. Cassel plays the best thief in the world. "I am very French in it. Varrry French. But I am not a baddy. That was the deal."

'Blueberry' is released on 23 July