Hollywood's diamond geezer

Isn't it a bit rich for a film star to start preaching about ethical consumer choices? "The movie isn't necessarily saying don't buy diamonds," he snaps back. "It's saying that every time you write a cheque for something, you are endorsing the way that company does business. When it comes to diamonds, I for one am going to get proof that it's not a conflict stone. Diamonds funded these warlords in Sierra Leone and four million people died. It's the awareness we have to have as consumers."

This is pretty spirited talk for a Hollywood actor - and all the more remarkable for one who once looked like such a lost cause.

DiCaprio spent the years immediately after the massive success of Titanic (1997) boozing and womanising. He made friends with Playboy boss Hugh Hefner forming the "pussy posse" with fellow wild boy Tobey Maguire. Though he vehemently denies they went under that charmless name, the strip clubs, wild parties and flings with Kate Moss, Eva Herzigova and Helena Christensen are well documented. The new millennium was greeted by DiCaprio in a satin dressing gown, smoking a cigar, apparently in tribute to Hefner.

The dreadful reviews that followed his succeeding filmThe Beach (2000), marked a low as the overpaid, over-partying DiCaprio piled on the pounds and became depressed. Reporters sent to interview him found a paranoid, hyperactive recluse.

When I ask, he's reluctant to go into details. He doesn't seem to regret those wild-boy days - and certainly doesn't sympathise with other young stars falling into the same traps as he did: "There are a lot more people going through a lot more terrible shit than becoming famous - so screw them!"

He is still into models, too, moving on from his five-year relationship with Gisele Bundchen to current squeeze, Israeli Bar Refaeli, 11 years his junior. But he seems to have found a sense of purpose, if not with his girlfriends, then certainly with his latter-day activism over the environment.

Ask him about it and he becomes as excitable as a schoolboy, talking about his hybrid car (from Tom Hanks to Cameron Diaz, they're all driving them in LA) and educating the new generation. He is, it seems, one of Hollywood's leading greens. He has interviewed Clinton on the subject he tells me, quizzed Al Gore about his lecture-tour and is producing and narrating a documentary called The 11th Hour, which focuses on the environmental crisis through the eyes of A-list activists like Stephen Hawking, David Suzuki and Mikhail Gorbachev.

Isn't he just jumping on the bandwagon? Isn't green the latest buzzword in Hollywood? "If people don't want to listen to what I have to say on environmental issues, that's their right," he says, shifting in his chair. "It's not going to stop me from using the opportunity as somebody in the public eye to highlight issues that aren't being given enough attention. The next generation - my children, my grandchildren - are the ones who are going to feel the hardest effect of what's going on in the world today."

It's tempting to believe him. He's a determined man. Born in 1974 to comic-book distributor George DiCaprio and his German wife, Irmelin, he faced a long struggle to become an actor. His parents divorced soon after he was born, and although they were supportive, he did it all under his own steam, from first audition aged six, to signing with an agent aged 15. "They were never classic stage parents," he tells me, "which was good because it can pose problems for child actors when they become adults because of that kind of drive from your parents."

So where did his drive come from? De Niro, it would seem. DiCaprio's first lead role in a film came in 1993 alongside the actor in This Boy's Life. "When I worked with him, it was the first time I saw somebody taking acting so seriously," he says. "I couldn't work out what was going on. He was asking the prop guy to change things four or five times. But I realised that those details were important to him and that those things make a good performance. He was a role model for me."

Perhaps thanks to De Niro's influence, DiCaprio started to take acting a little more seriously. He starred with Meryl Streep in Marvin's Room, and accumulated a swooning teen fanbase for playing Romeo in Baz Luhrmann's Romeo + Juliet. Then came Titanic in 1997 and more money than he knew what to do with.

While his dizzy ascent after his poor upbringing didn't do wonders for his ego, the financial security has at least allowed him to work with more or less who he wants. "I want to make movies that will be seen for generations to come, like the films that me and my friends used to admire," he says. "I don't think I've ever consciously chosen a type of movie so that the public looked at me as more of a man or a certain kind of actor."

And, thankfully, the films have improved as he's matured. He teamed up with Steven Spielberg on Catch Me If You Can and with Martin Scorsese for three films - Gangs of New York, The Aviator and The Departed.

This year's Academy Awards nominations are announced next week and he can be sure he'll appear on the list. Responsible, committed, up for an Oscar: DiCaprio would be nigh-on unrecognisable if it weren't for the model on his arm.

Blood Diamond is released on 26 January.