LEONARDO DiCaprio is lounging around Claridges in Mayfair, casually dressed in a grey jacket and a pale blue shirt with several buttons unfastened, trying to keep a foot on the ground. It's Tuesday afternoon and he's just heard that he's won a best actor Oscar nomination for Blood Diamond, a thriller with a conscience set in war-torn Sierra Leone. He's been nominated for Oscars twice before, as best supporting actor 12 years ago - he was only 20 - for What's Eating Gilbert Grape, and as best actor for Martin Scorsese's 2005 The Aviator. So he's not allowing himself to get carried away.

"It's a nice thing to be recognised, it really is, " he says.

"To put a lot of hard work and effort into a role, how can it not be nice? But it's not something you strive for. The more I act the more I realise I have no control over it and no way of knowing how anyone will react to anything I do. It continues to mystify me."

Put his cautiousness down to the hysteria he got caught up in when Titanic turned him into a teen idol, with mobs of girls screaming wherever he went.

According to Ed Zwick, who directs Blood Diamond, "The Titanic romantic idol was as much a surprise to him as it was to the world." It's not that DiCaprio now regrets Titanic. It would be hypocritical of him to disown the biggest grossing movie in screen history, which continues to earn him millions of dollars year after year. "I want to say again just for the record that I'm very proud of that movie and anything I say about my decisions before or after Titanic doesn't mean that I'm not thankful for that experience."

He's replying to my suggestion that Blood Diamond and his performances in The Aviator and in Martin Scorsese's The Departed, which won a Golden Globe nomination earlier this year, mark a welcome return to the kind of darker roles that initially made his name. "Titanic was a departure for me, " he admits. "To tell you the truth, I wanted to experiment and try something different to the more off-beat independent roles I'd been doing. I wanted to be a part of a movie of the calibre and the magnitude of Titanic, I wanted to have that experience in my life.

What it taught me was that you can never manipulate people's attitudes about you as an actor or as a person. I think there's something transparent that comes out whatever you do."

Brought up in Echo Park, Los Angeles by his German mother, a legal secretary who divorced his Italian father - a distributor of comic books - when he was just one, he seems to have learned from her a sense of order, at least with regard to his career. "I've kind of had a consistent attitude about what I've wanted to do as an actor ever since I've started out, and that's been trying to take on - for want of a better term - more complex characters. It gives me more to do in the preproduction process. It gives me more to think about when I'm on set. It gives me more freedom as an actor."

He spent months in South Africa before filming Blood Diamond, hanging out in bars with ex-mercenaries who, like his character, were involved in perpetuating Africa's vicious civil wars by trading arms for smuggled diamonds. "I wanted to find their mind-set and absorb their bitterness, to become like them, " he says. "The character I play appealed to me because he was somebody who was learning to feel again, he was someone who was so scarred from the apartheid era that he saw the continent of Africa as a wasteland where everyone takes advantage of each other, turning a cold shoulder to the implications of their actions. He was technically a very difficult character to play because the accent was so alien to me, the culture was so alien to me, and the environment was so alien to me. I'd never spent more than a week in Africa in my entire life."

DiCaprio likens his compulsion to research characters to a nightmare he used to have at school of turning up for class with no clothes on. "Because as an actor you need to be prepared for the unexpected, " he says. "I've learned it isn't always what's on the page. It's about those moments where an actor does something completely abnormal and you need to be able to react to that. If you just know your lines, it doesn't leave those opportunities open. You need the ability to know your characters enough to be able at any point to embrace accidental moments and be secure enough to have something to say out of the blue to the other actor."

Ed Zwick didn't write Blood Diamond for DiCaprio, but he was his first choice once the script was finished. "He's like Henry Fonda playing the bad guy in Fort Apache or John Wayne in The Searchers or Michael Douglas in Wall Street, " Zwick says. "There's always this great moment where an actor relishes the opportunity to subvert the inherent sympathy his persona generates and do something that's antipathical to that. The interesting thing is how they always maintain some transparency, some ability to see inside those characters. You see it in how Forest Whitaker in The Last King of Scotland is a monster and yet his vulnerability - his childishness and his fear - is also palpable. Those are the great villains. They're not only monochromatic. They're textured."

It's not just the character DiCaprio plays in Blood Diamond who is changed by the experience, he himself was too.

"It changed all of us. The thing about Mozambique, where we did most of the filming, is that everything is lived in front of you in the street, disease and death, sexuality and spirituality, everything is there before you. There's a certain detachment when you watch images on TV in the comfort of your western home and even when you send a cheque. But it really gets to you when you actually go there and see the orphanages and see how setting up a school can give a whole new lease on life to 300 children who lost their parent because of Aids. You can't help but be affected by it. Since then I'm trying to get more involved with issues in Africa."

Like Zwick, he's angered by the way Hollywood stars such as Beyonce are being encouraged by diamond lobbyists to counter the bad press generated by Blood Diamond by flashing diamonds at the Oscars next month in return for a payment of $10,000 to a charity of their choice. "I think it's particularly distasteful that the campaign is called Raise Your Right Hand when you remember that the rebels dealing in smuggled diamonds in Sierra Leone used amputation to terrorise people, " says Zwick.

Whether DiCaprio wins an Oscar at his third attempt or loses out to the favourite Forest Whitaker, he'll continue to go his own way. His next movie The Eleventh Hour is not a drama but a documentary on global warming.

"I've helped produce and write it, and I'm also narrating, " he says. "Al Gore was actually the guy who introduced me to the issue eight years ago. We pick up where he left off in his documentary An Inconvenient Truth, which has been nominated for the best documentary Oscar.

I'll be rooting for him at the Academy Awards on 25 March."

'Blood Diamond' is in cinemas now and reviewed on Page 14