HAVING TRIUMPHED at Cannes, worked with Brando and appeared infilmsasvaried as The Big Lebowski and Harry Potter,DavidThewlishasalready bankedmorecredibilitythanmost actors see in a lifetime. He's even survived Basic Instinct2withhisreputationintact,anot inconsiderable feat under the circumstances.

"Yeah, it's a pile of f***ing shit," says the 44-year-old, prodding an uneaten sandwich in a plush London café. "I did it for the money. Sharon Stone's a pain in the arse."

The way he tells it, however, his acting achievements are slight compared to the one we're here to talk about: the novel he started writing in 1999 and which has, finally, been published.It'scalledTheLateHector Kipling - a satirical and very funny journey through the world of contemporary art.

Though Thewlis has always written - songs, poems, journals, screenplays - having a finished novel fulfils a lifelong itch that acting has never been able to scratch.

"I don't think you can be a genius actor. It's not that hard for a start. You sort of wonderwhyanyone can't do it."

Genius, he thinks, is something else. It's painting, it's writing,it's sitting with Billy Connolly and Robin Williams in a bar late at night, as Thewlis has done, as they embark on a freeform comedic mash-up.

"I didn't get a f***ing word in," he says fondly as he recalls the event. "I had to leave the room because my head was hurting so much. And when you see something like that you know there's as much genius to it as there is to Einstein or Picasso - two people just talking but saying so many funny things per minute."

Connolly has contributed a kind quote to the dustjacket. So too has Jake Chapman, another friend, who has this to say: "I laughed and laughed until I read my own name amongst the carnage of Thewlis's characters. This book is a disgrace."

Hector, Thewlis's titular anti-hero, is a painter whose life starts to fall apart when his old friend Lenny Snook is nominated for the Turner Prize and starts to busy himself with a new conceptual piece for the Tate. A sofa with a bit cut out of it.

As if professional jealousy wasn't enoughtocontendwith, Hector'sownmoremodest exhibition is scuppered when a motorcyclist rides through the middle of one of his portraits as it's being loaded on to a lorry. Hector does heads. Big ones.

So, filled with self-doubt and self-loathing - and wondering if big heads aren't an artistic cul-de-sac in these days of aerated sofas - Hector takes up with sadistic psycho-goth Rosa, despite the fact that live-in girlfriend Eleni is in Greece visiting her dying mother. It's a cowardly and caddish thing to do but, well, that's Hector.

Some characters in the novel are real - David Baddiel, for example - while the rest are sharply observed composites of people Thewlis would see around Soho, where he lived when he started writing the novel, or in trendy Shoreditch, where he lives now and where he finally finished it.

The dialogue, meanwhile, is fast, funny, and features lines like: "How much of yourself do you think you could eat before you pass out?" It's the sort of thing Lenny and Hector like to discuss after one too many beers in Soho dives. Thewlis too, he admits.

Sowhythedelaybetween startingthenoveland publishing it? The project stalled on the day he finally completed the first draft. "I met Anna on the night I finished the first draft of the book," he says. "I typed The End' and went out and celebrated. She was out too. I got together with her. I fell in love and stayed indoors for months and months and months. My agent and my publishers were very patient. But years went by."

Anna is former Brookside actress Anna Friel, mother of Thewlis's two-year-old daughter Gracie and star of US television drama Pushing Daisies. So home, for Thewlis, is currently LA, where they live in a rented house in Beechwood Canyon, next to the Hollywood sign. He is, he admits, a full-time dad.

Thewlis has no plans to give up acting completely, however, which will come as a relief to the makers of the Harry Potter films. He returns as Professor Remus Lupin next year in Harry Potter And The Half Blood Prince. So far, he's pleased with the films. What, though, did he make of the author when he met her? "She was really lovely," he says. "She said she was very happy I was playing the part. Actually, she said Lupin was her favourite character. I said, Well put him in it more then.'"

He has also just finished a film written and directed by novelist Paul Auster, a hero of his. Thewlis embarks on a long and involved story about how his participation in the project came about. It starts in Cannes in 1993, with Mike Leigh introducing him to Auster when Thewlis was there with Leigh's award-winning Naked, and ends up with Auster ringing up Thewlis and asking him to be in a film. The conversation was weird.

"Hesaid,Doyouremembermeetingin Cannes?'" recalls Thewlis. "I said, I do, and it's very prescient you should ring because Mike Leigh said I should write a novel when he saw me shaking your hand.' He said, Did you ever write it?' and I said, I'm just finishing it now.' He said, I'll tell you why I'm ringing: I've written a film and I want you to play the main part. It's about a man who's just finishing a novel.' So I go, You're f***ing joking?'" The next thing Thewlis knew he was on a film set in Portugal with Swiss actress Irene Jacob shooting The Inner Life Of Martin Frost. Even weirder things were to follow.

"Paul said, I'm going to do lots of shots of you typing, so why don't you just carry on with your book?'" Thewlis polished his earlier drafts of the final chapter in the evenings, in the company of Auster. "About two paragraphs were actually written on set, on camera," he says.

Thewlis is full of stories like that. Some he saves for the pub, I imagine, but some he has written down, such as his mini-memoir about working with the late Marlon Brando and director John Frankenheimer on The Island Of Dr Moreau in 1996.

He became friendly with Brando on set and the pair would retreat to the screen legend's trailer to play chess. Brando, says Thewlis, was "quite mad He'd get changed in front of me endlessly. So I've seen him naked and he was a big fella."

It wasn't a happy shoot. Val Kilmer was going through a divorce, the original director had been fired, the French government was detonating nuclear bombs near the atoll owned by Brando whose daughter, Cheyenne, had committed suicide just months earlier. When Brando received news on set of another death in the family, he finally burst into tears and it was left to Thewlis to console him.

"If someone had told me three weeks earlier I'd even meet Marlon Brando, let alone be in a trailer hugging him and feeling his tears on my neck I wouldn't have believed them. But life is very strange."

As he speaks, Thewlis acts. He holds his arms out wide, to indicate Brando's bulk, and drops his head as if onto the older man's shoulder. Perhaps deliberately it comes out as a sort of crucifixion pose.

Thewlis's memoir of his time on the film set runs to about 60 pages and is currently with The New Yorker magazine, the editors of which are trying to persuade him to let them publish part of it. If it's as funny in print as it is over an egg and cress sandwich, it's worth keeping an eye out for. And with Brando and Frankenheimer both dead, there's only Val Kilmer left who could reasonably sue for libel.

"I'm not sure about it though," says Thewlis. "I don't want to get a bad name in Hollywood as the guy who makes a film then slags everybody off. But it is funny. I'm quite vicious about Frankenheimer, but he f***ing deserved it. He was an ugly, f***ing nasty, bigoted old git. And you can quote me on that."

So I have.

The Late Hector Kipling is out now (Picador, £16.99)