Robert Downey Jr.'s is not the first album from a Hollywood star, but the former hellraiser has produced one of the best.

Robert Downey Jr.'s slow crawl to adulthood has been harder and more public than most. A jailbird and bad boy even by Hollywood's sybaritic standards, the American actor turned 40 earlier this month, certainly older and probably wiser.

The excesses - even cigarettes and coffee - have gone. Instead, he has poured his energies into his debut album, The Futurist, on which he has written eight of the 10 songs. From Robert Mitchum attempting calypso in 1957 to Juliette Lewis's recent embarrassing kiddie-punk, actors and albums have made uncomfortable bedfellows. Downey Jr.'s effort is better, not because it sounds more like a Sting album than the last actual Sting album, but because it doesn't sound like it was made by an actor.

Embracing jazz-lite, some surprisingly thoughtful lyrics and, in the title track, an exceptional song, it establishes Downey Jr. as a genuine, if not great, music talent.

He refuses to remove his leather jacket, drinks tea as enthusiastically as he used to smoke unfiltered Camels and complains of jet lag. Some, though, might suspect his lethargy is more a result of spending the previous evening as a guest on Jonathan Ross's show, before carousing at a Pamela Anderson party and concluding proceedings at Tramp with Duran Duran's Nick Rhodes at 7am.

Aged five, Robert Jr. delivered his first line on screen ("Got any hair on your balls?") in Pound, a film directed by his father, Robert Sr., featuring humans playing stray dogs. Downey Sr. may not have been the perfect father, but he did introduce his son to music.

"Living in Greenwich Village in a loft with my mom, dad and my sister, I remember seeing a TV commercial saying Keep New York City Clean and I was really engaged by how cool the piano lick sounded. When my dad was writing, he'd play Coltrane and Mingus and Art Pepper and Jefferson Starship and Aretha Franklin. But he would just put an album on repeat. We'd hear Let It Bleed literally 15-20 times a day. Lemme tell you, that really seeps in when you're a kid."

As Downey Jr. hurtled towards adolescence, the family moved to Woodstock in upstate New York. Music soothed the loneliness. "At that point I was introverted. On my own one day, I went to the piano in the corner and started playing that Keep New York City Clean commercial." He became proficient without taking a single lesson. Extraordinary. "I know," he shrugs mock-modestly. "It happens. Would it have been nice if somebody had invested in a piano teacher for me? Yep. Would I now have more prowess? Maybe, but maybe I would have burned out if it had been shoved down my throat."

Music became his biggest secret. Between films, he tinkled away on his $200 keyboard. Post his role in Ally McBeal, but with current fiancée, film producer Susan Levin, as a stabilising influence, Downey Jr. plunged into recording.

"A major fricking undertaking," he shudders. "I'm used to coming to work and the call sheet saying, '7am, scene 14b, this costume'. Then they say 'action' until you're done. The autonomy and self-agency necessary to do a record were new to me. It felt more like the responsibility of being a director who was also in his own movie. Then I figured if people didn't like it I could always use another dose of humility. Instead, people are pleasantly surprised it doesn't suck."

Those poring over the lyrics looking for Downey re-examining his eventful past will be sorely disappointed. Take the song Details, about the decline of a long-standing affair. "You could say it was about my relationship with Sarah Jessica Parker," he reluctantly admits. "Does that song seem like an apology? Kind of. But that song is about a guy and a gal reincarnating over different periods of history." He looks quite pleased with his obfuscation.

Considering his prison terms were far from cushy and he claims to have "gone broke several times" while recording the album, the most surprising aspect is its absence of bleakness.

"It's an encouraging album," he states firmly. "I've been accused of being dark in the past, and with good reason, but it's not the way The Futurist turned out. There are some dank and hopeless songs left over, but I don't know if I really relate to that any more: you can't get through crap unless you're an essentially hopeful person."

While he admits that The Futurist is partly a response to being "a little disillusioned" with acting, Hollywood has forgiven and embraced him. He has five major films on his crowded horizon, from the Disney remake The Shaggy Dog to an adaptation of Philip K Dick's novel, A Scanner Darkly. What would really put him "on point", however, would be if the album were so successful he could justify a tour. The precedents augur ill: Bruce Willis took to the road to destroy some soul classics in the mid-1980s and, a decade later, Keanu Reeves's Dogstar were unwatchable. But stranger things have happened.

"Oh, I'd be up for it," he trills, positively childlike. "I would take three months off to do a European tour. Can you imagine that? It would be one of the most rewarding things I ever did."