Alison Jones hears Leonardo DiCaprio explain why working on Blood Diamond has changed his priorities

When Leonardo DiCaprio's name was announced as one of the Oscar nominees for best actor for his performance in Blood Diamond, it was reward for a year in which he has given flesh to not one but two complex and conflicted characters.

The first was Billy Costigan in The Departed, the undercover cop cracking under the strain of living a double life trying to win the trust of Jack Nicholson's charismatic crime lord.

The role for which he has been nominated is Danny Archer, an initially cold hearted ex-mercenary from Rhodesia (now Zimbabwe), whose long dormant conscience is reawakened by fisherman Solomon's (played by fellow Oscar nominee Djimon Hounsou) quest to rescue his son, a child soldier.

The smart money says that the Oscar will go to Forest Whitaker, who has already secured a Golden Globe for his towering portrayal as Ugandan despot Idi Amin in The Last King of Scotland. But Leo, a veteran of two previous nominations (for What's Eating Gilbert Grape and The Aviator), has his game face ready and has adopted the Olympian philosophy that it is not the winning but the taking part that counts.

"It is a pat answer but it's a nice thing to put a lot of hard work and effort into a project or a character like this and for it to be recognised.

"It is certainly not something I expect by any means or that we strive for during filming. The more I acted, I sort of realised I had no control over how people will react to any movie I do. If every actor and every studio had that magic formula we would all be making critically-acclaimed, multi-billion dollar hits every time."

The ceremony itself will place the Blood Diamond stars in something of a dilemma.

The red carpet is an annual bling-fest with the A-listers, particularly the women, dripping in borrowed jewels. However, working on this film has raised the awareness of all those involved to the terrible price that has been paid by many people working in the diamond trade.

Initially meant to be a conventional action adventure about two men searching for the same rare gem, director Edward Zwick decided it should evolve into an issue-based drama about the corrupting trade in conflict or blood diamonds, the sale of which has been used to pay for arms in countries at war.

The action is set in Sierra Leone in the 1990s.

In 2002 The Kimberley Process Certification Scheme was established to monitor the trade in diamonds and prevent the sale of conflict stones.

A voluntary system, it has undeniably improved the situation, however, the message to consumers that they must insist on learning the origin and history of any diamond purchases is one that must be continually reinforced.

"I have bought diamonds in the past, before learning about conflict diamonds and their devastating impact on places like Sierra Leone," admits Leo.

"I was pretty unfamiliar with the ramifications of some of the events that had gone on. But if I ever did buy a diamond again, I would make sure it was a conflict-free diamond and get it certified by the dealer I was buying it from."

A passionate environmentalist, the 31-year-old Californian insists his crusading nature will compel him to pass on what he has learned to anyone willing to listen.

"I am not going to stop talking about conflict diamonds and that issue but it depends when and where I am asked. We attach ourselves to certain causes and mine have

>> Listen to Leonardo di Caprio and members of the cast and crew discuss how making the film changed their attitudes

traditionally been environmental and I will continue to work on that, but that doesn't mean I am not going to continue spreading the word whenever I can on the issue of conflict diamonds.

"This movie changed all of us. We live in a very fortunate environment in the western world. To see the way the majority of the population live makes you realise how thankful we have to be and how little we have to complain about when people are striving for the bare fundamentals of life, like clean water and food.

"I am trying to get more involved in issues in Africa and I'm aggressively looking for more things to do and doing research on what I want to contribute to next."

He was deeply moved when he and the rest of the cast met with young children from the SOS Children's Village in Maputo in Africa (SOS Children is the world's largest orphan and abandoned children's charity).

When filming finished props, construction materials, costumes and even personal belongings were distributed to local orphanages and hospitals. The construction team also volunteered to make desks and chairs for the schools and a Blood Diamond Charity Fund was established with contributions from the actors and crew to help the communities who had played host during the production.

"There is a certain detachment you have when you watch images at home on television," says Leo. "Even if you do send a cheque."

"If you actually go there and see an entire school set up and people taking care of 300 children that have lost their parents because of Aids and how they have a new lease on life, that's pretty amazing and makes you want to do more."

Both the role of Billy and Danny reflect Leo's growing maturity, though physically the goatee he sports does little to disguise the rounded, babyfaced features that meant he still convinced as a teenager less than five years ago in Catch Me If You Can.

He endured rather than enjoyed a period of Leo-mania following his elevation to global pin-up when he went down with the ship in Titanic (his haircut was actually banned in Afghanistan by the Taliban).

Together with his best friend Tobey Maguire and a small entourage who became known as the Pussy Posse, Leo raised a little hell and notched up an impressive list of sexual conquests.

But a long relationship with supermodel Gisele Bündchen seemed to domesticate him. He became politicized and returned to the edgier work for which he first became famous.

"For the record I do want to say I am very proud to have been part of Titanic but it was a departure for me. I'd traditionally done more independent darker films but I wanted to try something different and be part of something of that magnitude.

"You can never truly manipulate peoples' attitude about who you are as an actor or as a person by decisions you make but I have been pretty consistent since I started out in wanting to take on more complex characters."

Leo's Danny Archer is a dispassionate observer of the suffering he sees around him, initially only interested in how it might benefit him. Gradually his latent humanity is nurtured by the passion he sees, not just in Solomon's struggle to reclaim his son but also by crusading journalist Maddy's (Jennifer Connelly) mission to expose the story behind conflict diamonds.

A dedicated researcher – Leo likens his fear of being unprepared to being "like that nightmare of showing up at school with no clothes on" – he spent time with former mercenaries and soldiers of fortune, try to nail the South African accent and also to study their detachment.

"There is a hardened shell that surrounds a lot of these guys. I incorporated the line 'You Americans love to talk about your feelings' from my experiences hanging out with some of these South African oaks because it is very hard for them to divulge anything about their attitudes towards Africa, their mixed emotions to the politics or their experiences at war.

"It did take a certain amount of taking them out to various bars and getting them drunk, rehashing past demons and that was some of the most beneficial stuff for me. It shaped my character and helped me understand some of the emotional turmoil he had gone through."

For his next project Leo is taking a break from fictional drama and concentrating on the global one.

He has been interested in green issues for a number of years and actually campaigned on them for John Kerry, giving a number of speeches about how the Bush administration had damaged the environment.

He has now helped write, produce and narrates a documentary called The 11th Hour.

"It has aspects of Al Gore's An Inconvenient Truth. Al Gore was actually the one who introduced me to the issue of global warming eight years ago and he has really made the issue a household name now through An Inconvenient Truth. It is pretty revolutionary what he did, compiling decades of research.

"The 11th Hour film is going to focus on 60 other environmental leaders. People like Gorbachev, Stephen Hawking, David Suzuki (Canada's foremost environmental activist), experts in every field representing every eco-system, including global warming, and giving their input on the environment uncensored and without having to argue whether it is a reality or not"

>> Mike Davies reviews Blood Diamond, along with other new releases Bobby and Venus

>> Listen to Leonardo di Caprio and members of the cast and crew discuss how making the film changed their attitudes