Man of the world: Leonardo DiCaprio

If yesteryear's Hollywood leading man was merely content with saving the world on-screen, then today's film idol must be seen to be saving the world off-screen, too. But this won't be news to anyone who has followed Leonardo DiCaprio's environmental campaigns over the past eight years, putting him at the forefront of a new breed of politically aware actors whose numbers include Brad Pitt and George Clooney.

Admittedly, DiCaprio had fun on the way to his present reincarnation, indulging in all-night poker parties, jet-setting and hanging out with supermodels. But today he's far more likely to be spotted cycling with girlfriend, Israeli model Bar Rafaeli, or campaigning. Not that the actor could be accused of jumping on the environmental bandwagon – the state of the planet has been a pet topic since he was a young boy with dreams of becoming a marine biologist.

But fate intervened and at 15 years old he would become a TV teen idol on US sitcom Growing Pains; making his inauspicious movie debut a year later in Critters 3: You Are What You Eat. Two years later, a role opposite Robert De Niro in This Boy's Life garnered much critical acclaim, and set him on course for the starring role in one of the biggest movies of all time, Titanic.

"I'm very happy doing what I'm doing, I've gotta say. I love to keep busy," he says. "It excites me and it makes me jump out of bed in the morning to be able to do this stuff." He wasn't always so happy, particularly in the wake of the phenomenal success of Titanic. DiCaprio then starred in The Beach, Danny Boyle's adventure thriller about a hippie island paradise with a sinister secret. Filmed in Thailand, the experience sowed the seeds of DiCaprio's eco-consciousness.

"Titanic began a period of rebellion for me. Everything happened so quickly, I began to feel engulfed by it. As soon as enough people give you enough compliments and you suddenly have more power than you've ever had in your entire life, it's not so much that you become an arrogant prick, or you become rude to people, but you get a false sense of your own importance. You're treading in dangerous territory when you begin thinking you've actually altered the course of history. It was like a runaway train. I didn't understand what was happening to me."

Although wary of the media, he's savvy enough to use it to promote his causes, particularly in the case of The 11th Hour, a documentary that he produced, narrated and co-wrote.

Today, he is wary of sounding like he's preaching, and deflects discussion of his own green lifestyle: "Yes, I have solar panels and all that sort of stuff. However, the more I learn about the subject matter, I also realise that people find it sort of patronising. They feel like they're being told what to do when somebody like me talks about the fact I have solar panels. Not everybody can get solar panels and not everyone can drive a hybrid car. And, to me, it's not about putting blame on anyone or telling anyone how to live. It's just about saying, 'Let's all be aware of these issues', so the next time we buy something we realise where that product comes from or how that company does business. It's just a publicity game for the environment at the end of the day."

Born and raised in Los Angeles, DiCaprio began attending auditions aged 10, and appeared in numerous TV commercials. He has claimed that his parents never tried to steer him towards a Hollywood career: "We're all after love, aren't we?" the 34-year-old actor recently told Parade magazine. "Love is what people are hungry for. That's absolutely why I became an actor."

If the love affair with his fans soon became one-sided, DiCaprio would discover a more meaningful love of acting through his collaboration with Martin Scorsese. Supplanting Robert de Niro as the director's go-to leading man, he starred in Gangs of New York, The Aviator and The Departed and, next year, makes his fourth film with the director in Shutter Island, a Fifties thriller based on a novel by Dennis Lehane.

Another of his mentors is Ridley Scott, with whom he has made the political thriller, Body of Lies. It marks a reunion of sorts with Russell Crowe, with whom he last co-starred 14 years earlier in The Quick and The Dead. Recalling his first meeting with Crowe, he says: "Well, I was 18 at the time so there was a difference in our ages, but we were both in the same sort of position where the people above us in the cast were Gene Hackman and Sharon Stone and everyone was looking at the two of us going, 'Who are these guys?' So that naturally put us together in a way where we'd just hang out together because we didn't care about status."

In Body of Lies, based on the novel by David Ignatius, who covered the CIA and Middle Eastern affairs for The Wall Street Journal, DiCaprio plays an undercover operative in Jordan reporting to Crowe's overweight CIA boss. Kidnapped and tortured by terrorists, DiCaprio gives a remarkable performance.

Next up he reunites with Titanic co-star Kate Winslet, as a husband and wife in Fifties romantic drama, Revolutionary Road, directed by Winslet's husband, Sam Mendes.

"Kate has remained one of my closest friends and is the best actress of her generation. She brought this book to me, which was a portrait of America after the war and a family trying to become the idyllic image of what a family is supposed to be. These two people are struggling to do that and they're torn apart because they feel like they've lost their identity. Kate and I knew we could push each other's buttons, performance-wise, and knew we could pull stuff out of each other. We've known each other since we were almost teenagers. So it was something that I jumped at the opportunity to do.

Today, dividing his time between his film career and environmental campaigning, partying "like it's 1999" in the chilly economic climate is no longer an option: "In terms of the environment and our country shifting to alternative technologies, I've been profoundly disappointed for years. We should have started eight years ago to be less dependent on foreign oil and start to invest in some of these new technologies, but now we're way behind the curve again and the United States should be the one to set an example for the rest of the world. These last eight years we've failed miserably; we're the biggest polluters; the biggest contributors to waste."

Earnest though he is, some of DiCaprio's fans are preoccupied by the fact he remains one of Hollywood's most eligible bachelors. Having enjoyed romances with Demi Moore, Juliette Lewis and supermodel Gisele Bündchen, has been dating swimsuit model Bar Rafaeli for the past three years. Witnessing his friends Tobey Maguire, Brad Pitt and Matt Damon becoming parents, he admits: "I see these guys with their families, and it reaffirms for me that I definitely want a kid someday. I absolutely believe in marriage," he says. "My parents were divorced before I was even born, but that's never bothered me. They're both real cool people, and I've enjoyed sharing both their lives rather than having one life."

'Body of Lies' opens on 21 November