WAFTING HER cigarette smoke out through the open hotel window, Charlize Theron has a point to make. "I'm not a politician," she says, "I'm an entertainer." In other words, the statuesque South African, who has recently become a US citizen, knows all too well that spouting her beliefs can spell trouble, as a press conference in Venice for her latest film, In The Valley Of Elah, taught her.

In The Valley Of Elah is written and directed by Paul Haggis, who made the Oscar-winning Crash. In it, Theron plays a detective helping a taciturn Tennessee father (played by Tommy Lee Jones) look for his son, who is missing after a tour-of-duty in Iraq. Theron is keen to stress that the film is not anti-war but deals honestly with the consequences of sending men and women into battle. "You can't expect them to come back and not be traumatised," she explains. "My opinion is that our president should be a little bit more responsible. The thing that I would say this film is really about, is when we watch the news, we see those politicians in their very expensive suits and air-conditioned buildings telling us how our soldiers are doing in Iraq. And I think this film is the voice of the soldiers that we're not hearing in journalistic news today."

In some ways, the 32-year-old must be secretly pleased that she's being asked to talk politics. She arrived in Hollywood after a career as a ballet dancer was cut short through injury and her blonde hair and lithe physique automatically saw her cast as eye-candy in films like The Devil's Advocate and The Astronaut's Wife.

Then came Monster where - to play serial killer Aileen Wuornos - she gained 30lbs and wore dental prosthetics. It won her the Best Actress Oscar in 2004 and had critics and audiences alike marvelling at the physical transformation. The real metamorphosis, however, has come since. Gaining a second Oscar nomination in 2006, this time for her role as a victim of sexual harassment in Niki Caro's film North Country, Theron can now be considered a genuine Hollywood heavyweight.

Her one post-Monster aberration aside - the disastrous sci-fi Aeon Flux - Theron has been very careful with her career, even turning down a role in the Bond film Casino Royale to ensure her industry kudos remains high. "It's different now," she muses. "In my 20s, I had to walk into the room like a cannonball. I don't do that today. But that hunger is still there to find that challenge to do something that is intriguing and different."

Refusing to act for 18 months after North Country because nothing interested her, she concentrated instead on her company Denver And Delilah Productions. She produced 2006 Cuban hip hop documentary East Of Havana - even bringing it to the Edinburgh International Film Festival - and forthcoming family saga Sleepwalking, which makes its bow at the Sundance Film Festival this month.

More interestingly, Theron is currently in talks with South Korean auteur Park Chan-wook over an American remake of the final part of his bloody Vengeance trilogy, Lady Vengeance.

"I'm fascinated by somebody like that, who has been obsessed with his theme of vengeance. I picked his brain," she says. "In a way, Aileen Wuornos had a theme like that. Any time that you can take a subject matter that people don't really want to know about, that's uncomfortable, that's always challenging."

Of course, if you know anything about Theron it is that her childhood was overshadowed by a single ghastly event: the shooting dead of her abusive father by her mother. The killing occurred at the family farm in Benoni, outside Johannesburg, and no charges were brought as the shooting was deemed to have been in self-defence. Theron was 15 at the time and witnessed the event.

Naturally it's not something she talks about in interviews and she denies that there are any parallels between her past and her recent work, with its themes of abuse and violence. "Anybody's formative years have a huge impact on their life," she shrugs. End of story.

She even plays down the influence a childhood spent in apartheid-era South Africa might have had on her choices of role. "I am from a country where that violence is very evident but it is not a deciding factor in choosing my work. It's not a huge influence."

Yet it's still easy to see why Theron is now so motivated to talk about political issues through her work. "I was raised in a very politically-aware family," she explains. "I was raised from a very young age with an awareness that apartheid was wrong. My family was very active in being a part of a society trying to stop that, and we all knew that it was just going to be a matter of time."

Her current thirst for socially aware material has seen her take a role in the forthcoming Battle In Seattle, a true story set during the five days of rioting which overshadowed a 1999 meeting of the World Trade Organisation there. The film marks the directorial debut of Theron's long-term boyfriend, Irish actor Stuart Townsend, who she met on the set of 2002 thriller Trapped. She stars opposite Woody Harrelson and calls the film "a really beautiful ensemble kind of like a Crash".

Theron lives in Los Angeles with Townsend, to whom she considers herself married in all but name. Last year, finally, she gained US citizenship, giving her dual nationality. It's something of "huge significance", she says. "I've lived half my life in South Africa and half my life in America. I feel like that's my home right now. I want to be able to have a voice. I want to be able to vote. I want to be able to be a part of the decision-making process - albeit on a minute scale."

In The Valley Of Elah is out now