"I'm just on the phone to dad!" says Robert Downey Jr., settling me down in one room of his vast Mayfair hotel suite before disappearing to finish his call. John Kenyon, the actor's muscular trainer/friend, makes some herbal tea in the meantime, and when Downey reappears we're deep in conversation. "I'm glad you've met my boyfriend," he says, grinning gleefully as Kenyon almost chokes on his tea. "I do hate those actors who introduce their lovers as their trainer or something." His friend recovers enough to laugh, but seems worried that my tape was running at the time. "It was a joke," he says, several times during the evening. "You know it was a joke?"

Robert Downey Jr. is in London to finish his first big-budget action movie Air America, in which he co-stars with Mel Gibson. An actor that has always threatened to become big box office, Downey has never quite made it despite a string of respectable brat pack movies (The Pick-Up Artist, with Molly Ringwald; 1969, with Kiefer Sutherland), some critically-praised, more mature roles (Less Than Zero, True Believer), and various appearances in his film-maker father's avant-garde black comedies. Air America, which opens here this month, is a buddy movie with a vaguely moral background, in which Downey and Gibson play a pair of wise-cracking pilots caught up in the secret war the CIA waged in Laos in the Seventies, a war funded by US-aided drug runs and kept hidden from the public by Nixon's lies. When we met, the location shots had already been completed in three gruelling months in Thailand, and the crew had moved to Britain's Shepperton Studios to tie up the close-ups and technical shots.

"I'm not going to be doing a big action movie again for a long time," says Downey, who doesn't seem to have enjoyed the experience. "Unless it's something that really has something to say. No one gets killed gratuitously in this film. It's not like one of those bang, bang, bang bullshit things... But it's just so tedious! It's acting, but the acting comes from just bearing it, getting through all the technical shit as opposed to interacting with someone, taking some chances, making some huge mistake - doing anything except jumping out of airplanes."

"Nowadays, there are no more Clark Gables and Clark Kents. You've just got to do what feels right," he says, citing the example of his Less Than Zero co-star James Spader, who picked out a little left-field script called Sex, Lies and Videotape and suddenly found himself lauded at Cannes. "Ultimately, you look much more stupid doing a movie that you thought was going to make you and it bombs than doing something for $1.500 that had a feeling."

Downey's first break came when he and a friend, Anthony Michael Hall, landed a slot on Saturday Night Live, the US comedy/satire show that launched the likes of Dan Aykroyd, John Belushi, Eddie Murphy and Chevy Chase. "I don't remember much about it," confesses Downey, who was soon on a drinking and chemical binge that almost matched the excesses of his character in Less Than Zero. "Anthony and I said, 'We want our own office - the one that Belushi and Aykroyd had.' Then we found out that they used to have bunk beds so we said we wanted some too. Three days later, these guys brought up bunk beds. It was weird. Like, the show finished at 1 am on Saturday night and there would always be a party somewhere."

Such on-the-record confessions are inevitably followed by the clean-up story, and this is no exception: hence the trainer, who not only keeps his charge healthy but is also collaborating with him on a script. What is left now from the days of live TV improvisation is a constant stream of wisecracks, What do you do when not making films, I ask (a dumb question, admittedly, but the kind of thing American actors usually seem to warm to). "I smoke crack and eat faeces. I mate with aliens and stuff pieces of young boys into refrigerators," replies Downey, deadpan. "No, I work out, I run. I'm really together now. Two years ago I would have had to leave the room four times during the interview to inhale some illicit substance. That's not what I'm about any more."

What he's about now is being politically active, something encouraged by his girlfriend of six years, actress and activist Sarah Jessica Parker. He worked for Democratic candidate Mike Dukakis during the last presidential elections along with other young Hollywood names, he calls Jesse Jackson "the greatest orator of our generation," but admits his support for the Democrats is shaky: "What we need is a new party, but until then, I'll just go with the one that lies the least."

Since Bush won, he says, his politics have retreated to the personal: recycling his aluminium, investing in solar power, and being completely aware of how twee it all sounds. He's also left high-powered brat pack agent Loree Rodkin in order to manage himself, and plans to do more work with his father as well as write, produce and direct his own film. A case of rampant ego? "Not really. But if you just sit down and write a script with no telephones, no cars, no gratuitous love scenes, and where no one gets shot in the middle of the forehead, then you're on to something."