Leonardo DiCaprio has come a long way from being the 23-year-old heart throb who sparked teenage mania in the blockbusting tear-jerker Titanic.

Ten years on, he still has the same youthful good looks, which today are partially covered with a little beard, and he still draws a screaming crowd to his premieres, but he's a far more serious actor.

He campaigns about the environment, is politically outspoken and carefully chooses his roles.

When we meet to discuss his latest film, Body Of Lies, at a swish London hotel, it's just days after Barack Obama's historic election victory and Leonardo is obviously thrilled.

"I was in Rome [on election day], where I watched the 2000 debacle. I stayed up all night and watched the results come in and it was an overwhelming, resounding victory for our fantastic new president Mr Obama. I couldn't be more excited," he gushes.

Appropriately, Body Of Lies deals with the very real ramifications of the outgoing president's 'war on terror' in the Middle East.

Leo plays Roger Ferris, a CIA agent on the ground, spying on terrorist cells, while a fattened-up Russell Crowe plays Ed Hoffman, who plans Ferris' operations from his laptop back home in America.

"I liked this character a lot," the 34-year-old says, flashing that famed smile.

"In a turbulent time like this, when the US is occupying the Middle East, here you have a highly trained CIA operative who's very effective at what he does, he's forged valuable relationships in the Middle East but he's not looking for that quick fix, the appearance of victory, he's looking for long-term solutions and he's trying to respect the culture and reach a positive result.

"Meanwhile his country is constantly undermining him and usurping his intelligence and I feel like my character was operating in a higher moral context than his country would like him to and that's fascinating for me.

"He also goes on a long journey where he isn't beholden to any country or idealistic thought or political regime and he doesn't necessarily believe in the war. What a great character to be able to play certainly at a time like this."

You could almost believe that Leonardo was talking about himself or the new US administration.

"There's a lot of differences between the old administration and the new one," he concedes.

"I think Obama's been very vocal about wanting to pull out in the Middle East and stop the war there and the US has spoken in droves about that as well.

"What's been beautiful for me, going on this film tour, is to see how supportive the rest of the world has been about Obama winning.

"It's really a special time in my life to watch this historic event and it's been more profound that I ever even imagined."

Body Of Lies is the latest in a string of issue-led films from Leonardo, including Blood Diamond for which he earned an Oscar nomination in 2006, and it enabled him to work with the acclaimed director Ridley Scott for the first time.

He admits that the pace of the shoot in Morocco was 'exhilarating'.

"You have to encompass how much work Ridley has done throughout his career and how comfortable he is with his craft.

"So much of the movie-making process is waiting around for a director to figure out what they want and here you have a director that is so precise and trims so much of the fat out of the process that none of it is a waste of time.

"He believes each scene will serve its purpose and the end result is you walk away from making a movie probably a month and a half earlier than most filmmakers would make it in," Leonardo adds.

"Ridley trusts you and relies on you to know your character and he instils a confidence in you and it's a great working relationship."

Not only was filming intense, Leonardo's character had to be able to speak Arabic as well as dodge bullets and explosions, so the role required dedication and was physically very demanding.

"I did do most of my own stunts and Russell Crowe will attest to this, you do walk away battered and bruised, but it's still a lot of fun."

At one stage in the film, there is a graphic interrogation scene, which Leonardo insists is crucial to the film's authenticity.

"We knew it was a pivotal moment in the movie, that it wouldn't have the same intensity or realism unless that sequence was authentic as possible," he says.

"That was the scene I put the most thought and energy into. I talked to the ex-head of the CIA to say what would a person do in this situation.

"It culminated in a three-day really hard sequence shot in a tomb underground and I got a chest cold afterward," he adds.

Leonardo had first worked with Russell Crowe on The Quick And The Dead when he was just 18. The Australian actor apparently hasn't changed over the years despite all his success.

"We kind of forged a friendship on that set because we didn't know where to fit in, we didn't belong to the character actor group or the movie star group," recalls the actor.

"I remember talking to him a lot about movies back then, the type of actor that he wanted to be, the type of films that he liked and he's the same guy that he was back then.

"He's incredibly funny, he's committed, he's great to be around and I was happy to reunite with him on this movie, because I think he's developed an incredible body of work, he's really done some unforgettable performances."

In January, Leonardo will team up with his Titanic co-star Kate Winslet again in the Oscar-tipped Revolutionary Road, directed by her husband Sam Mendes.

But rather than recapturing the star-crossed lovers on the infamous sinking ship, Leo and Kate play a young couple in 1950s America, struggling to make their relationship work.

"She's one of my best friends and the truth is we've been in communication a lot since Titanic and this project sort of came out of nowhere, it was one she had secretly been developing," he explains.

"We both knew the territory very well and all the stuff that would come with us working together again, but this was a unique piece of material. For a relationship movie, this was diametrically opposed to what we had done before.

"It's a fragmentation of a relationship, the disintegration of two people that are battling to try and stay together, as opposed to..." Leo tails off smiling, as he realises the ironic comparison to Titanic's Jack and Rose.

But as Leo steers talk back to the more serious subject of the new US President, any nostalgia for his heart-throb Titanic days seems to be long gone.

ALL ABOUT LEONARDO

Leonardo DiCaprio was born in Los Angeles and grew up in some of its less glamorous districts.

His film breakthrough came in 1993 when he starred alongside Robert DeNiro as a troubled teenager in This Boys Life.

In recent years, Leo has turned his hand to producing and last year, he narrated and wrote the eco-documentary The 11th Hour.

He now lives the life of luxury in the Hollywood Hills, collects art and is said to be dating 23-year-old Israeli model Bar Rafaeli.

Leo remains unphased by fame, saying his frenzied reception at premieres is like an 'out of body experience'.

Please note: Leonardo DiCaprio turns 34 on November 11.

Body Of Lies is released in cinemas on Friday November 21.