Robert Downey Jr. is Charlie Chaplin. Sort of. Adam Sweeting reports.

An American critic thought that Robert Downey Jr. had "a kind of happy-go-lucky irony," which may have been a polite way of saying he behaves like a complete prat. Right now he's holed up in Mayfair's plush St James's Club while he dubs some dialogue on to his portrayal of Charlie Chaplin in Sir Richard Attenborough's biopic of the silent screen superstar, Charlie, out next month. The diminutive Downey is all but invisible behind a huge club sandwich. Between mouthfuls, he spouts stream-of-consciousness soundbites about life and art.

"I feel I gave this movie everything I could," he splutters. "Every fucking fibre in my body. As a matter of fact, I resent that I didn't give like, y'know, my high school years or my relationships the energy that I gave to this film. I don't know how Attenborough [he pronounces it Atten-burrow] fucking did it. I could do it, but not like he did. If I were an Indian tribe, he'd be the shaman."

Dear, darling, luvvy Sir Dickie, a shaman? Tripping and dancing round a campfire, naked and painted green? Incredible. But Downey is racing ahead: "On this movie I had a lot of experience with repetition of humiliation. I was sure I'd nailed it, but it was like, bang, you're wrong. As far as I'm concerned - no offence to anybody - everything I've done until now was a well-compensated-for joke. I know that this is a springboard into chapter two."

Downey has really got it bad. He's even convinced himself that Chaplin's movies were better than anyone else's. "Chaplin came to America," he spiels, "and turned an arcade novelty into an art form, single-handedly. People say, 'Buster Keaton was the real genius'. I'm like, go back to bed on that one. Chaplin is much more important than anyone ever involved in films."

"The worst Chaplin movie is better than the best anyone-else movie I've seen. I'm sorry, I don't give a shit about DW Griffith, I don't give a fuck about Orson Welles. I'm a little over the top right now. I'm real excitable because I'm really starting to realise what this experience was about for me. It was like someone saying, 'You have no excuse not to try after this. You have the ability, you have the information, you've had the experience, you've relived the life of someone who did it right'. Career-wise, Chaplin nailed it."

Let's leave Robert to blather away in the background for a few moments and consider how he came to be playing the lead in Sir Richard's prestigious £25 million super-flick. He was born 27 years ago in New York, the son of Robert Downey Senior, a 60s underground film-maker of some repute. Bob Snr's best-known effort was Putney Swope in 1969, while Pound, which he made the following year, gave Robert Jnr his first line of on-screen dialogue. He played a puppy in a dog pound which asks a fellow canine detainee: "Got any hair on your balls?" From such a little acorns Hollywood careers can develop in short order. In 1985/86 Downey Minor did a stint on TV's Saturday Night Live. He began to score movie roles, and his first efforts in Weird Science, The Pick-Up Artist and Less Than Zero offered him valuable insights into the real meaning of artistic failure. Downey's portrayal of a drug-addicted male prostitute in Less Than Zero gave him an ideal excuse to let his party animal instincts run riot and he ended up in an Arizona rehab clinic for a month.

"Ask someone what they don't want: are they prepared to kill themselves in the process of finding out what they really want?" is how Downey remembers the experience. "Well, no. Are you sure? Yes. You're wrong. Here's why..." Following all this? Me neither. "This whole planet has a dysfunctional relationship with everything because in some ways it's like we all want to alter our consciousness."

With Air America and Soapdish, Downey found himself putting down some serious career roots. He describes Air America as "a piece of shit," though he's glad it gave him the chance to work with Mel Gibson. And Soapdish paired Downey Jr. with Kevin Kline, who reappears in Charlie as Chaplin's best buddy, Douglas Fairbanks Snr.

Downey is buzzing with the Chaplin-ness of it all, but what does he make of criticism that Chaplin was maudlin, sentimental and pretty dumb all round? "Sentimental? Sure he was. For such an American wannabe-macho guy, I'm incredibly sentimental and I love that stuff. I love to be touched by things, I love to cry. Life is a sentimental business, it really is."

Fired by the vaste amounts of learning which playing Chaplin has given him, the bumptious Downey now intends to busy himself with writing and directing. He's nearly finished his own musical, The Child ("I think I write good music," he claims), which is supposedly "a romantic comedy with mythological undertones."

"Hey, I think I give a good interview, don't you?" he bounces, grabbing his trilby and setting off down to the shops in Bond Street.