The snakes she performed with still had fangs. Angelina Jolie tells Michael Dwyer about making 'Alexander' with Colin Farrell.

Stories abound about Angelina Jolie, and some of them are true. The tabloids are obsessed with her, just as they are with Colin Farrell, who, although only a year younger than Jolie, plays her son in Alexander. Put the two of them in the same film and the rumour mill inevitably goes into overdrive. It seems irrelevant that there's no substance to the gossip.

There has been more of the same since Jolie teamed up, after making Alexander, with Brad Pitt for Mr And Mrs Smith, which opens in the summer. As far as some media outlets were concerned, Farrell was past tense and Pitt was the perfect fit as the new man in her life.

Then again, Jolie has a larger-than-life off-screen persona that has provided ample fodder for the media. She doesn't turn 30 until June, but already she has been married and divorced twice. When she was 20 she wed Jonny Lee Miller, the young English actor she met when they co-starred in Hackers, in 1995, and four years later she went up the aisle with the truly idiosyncratic Billy Bob Thornton, who shared her interest in tattoos, blood and unusual rituals.

While all of this was chronicled in drooling detail, Jolie maintained a remarkable work rate, starring in 20 films since Hackers and winning an Oscar, at the age of 25, for her searing portrayal of a mentally disturbed young woman in Girl, Interrupted. And she has worked tirelessly for Unicef and as a United Nations goodwill ambassador, travelling to Cambodia, Chechnya, Chad and Darfur to draw attention to crises from mass poverty to the prevalence of landmines.

Her father, Jon Voight, won his Oscar for Coming Home when she was just three years old; much more recently she had a widely reported estrangement from him. Consequently, it is surprising to see Voight arrive at the Los Angeles hotel where I am to meet his daughter an hour later. It transpires that it is mere coincidence: he happens to be doing interviews to promote his own latest film, National Treasure. I decide not to note his presence when it is time to interview Jolie.

She looks terrific, which hardly comes as a surprise, and she proves charming and quick-witted. Oliver Stone, the director of Alexander, describes her as a spectacular young actress, adding: "A lot of modern actresses play the polite middle, but with Angelina you have more of the Bette Davis tradition. She goes for it in a strong, determined way, and it's rare to see that with young actors. They don't have that confidence. But Angelina had developed a strength that was just right for Olympias. You couldn't ask for a better match."

Jolie says she knew little about Olympias, the mother of Alexander the Great, before Stone offered her the part in his film. "Then I did a lot of research," she says, "and I found her very interesting as a mother to Alexander. She was tough, very tough. I suggested a few important changes in the script, but she was obviously a character that Oliver found very interesting, too, because he had a very strong mother and he respected the relationship between Olympias and Alexander."

When Jolie and Thornton were married, they adopted a Cambodian boy named Maddox, who lives with her. "I couldn't have played Olympias if I didn't have a son," she says. "I wouldn't have understood her complete focus on him. You know, she's not unlike how I would have been if I lived at that time.

"That was a time when women had no rights, when they were considered as the vessel that carried the child for the man. So she couldn't allow Alexander just to be happy and to do what he wanted to do with his life. It was all about survival, and she was tough but human, and deep down that came from her love for her son.

"There was also the responsibility she had to carry. If he did the wrong thing she'd be murdered, and a few years after his death his enemies stoned her to death on the street. It's funny, because people say that Oliver doesn't provide many good roles for women, but Olympias is unusually strong."

True, but Jolie and Rosario Dawson, who plays Roxane, the princess who marries Alexander, are the only women with substantial roles in the film.

When 170 men from the cast spent three weeks in the desert for boot camp in preparation for the movie, the two actresses were not invited. "I was so irritated they didn't ask me along, because I would have loved doing boot camp," Jolie says. "I like a challenge and I love horses and working with them, so Capt Dale Dye, who organised it, has promised me he'll give me my own boot camp. There's a camaraderie between the guys when they survive that together, and I envied the men in the film for having that experience."

Instead Jolie got to play several scenes with snakes draped around her neck and writhing at her feet, because Olympias, as a worshipper of Dionysus, was accustomed to being surrounded by snakes. "I bonded with the snakes," Jolie laughs, "and they became my friends and my accessories. They were amazing. They weren't defanged, because whoever owned them thought that would be inhumane. They fed them the day before we worked with them, so they wouldn't be hungry."

That must have been a relief. "Yes," she says, "but we did have moments, like the night when we did the scene where I was lying on the floor and screaming, and Oliver said: 'Where are the snakes? Where are the snakes?' They would drop these creatures all over me. I became quite comfortable with them.

"The only time I felt uncomfortable was when I had to put one of them around the six-year-old who was playing Alexander as a young boy. I was terrified the snake would get angry, because there was so much else going on in that scene. The poor kid, I felt sorry for him, because it was his first scene ever in a movie, and I was the responsible adult in this movie. It couldn't have been a crazier way for a kid to start on a movie."

It felt perfectly natural for her maternal nature to make her feel protective of the boy, whom she had never met before. She says she is seriously considering adopting a second child but denies reports that she was seeking a brother or sister for Maddox when she went to Russia last year. "I visit orphanages all the time, so people can get confused," she says. "When I was in Russia I visited children in hospital and I visited orphans. But I may well adopt a child in the next six months or a year. I would like to adopt a brother for Maddox, who would be around his own age, and that would be interesting. Or maybe a sister."

Jolie regards her activities as a UN goodwill ambassador as the positive side of celebrity, citing a recent trip to Darfur. "I can see myself making a lot of apologies as an American as I go around the world," she says. "For the last four years I've been going around saying, look, you don't understand the American people, but since the election that's really harder to defend, because the majority of the American people have voted for this president.

"I live in Europe, and travel through there and through Asia and Africa a lot, and I know that America is viewed very differently than people in America realise. I'm taking out Cambodian citizenship in a few months, although I will retain my US citizenship."

This week Jolie was reported as criticising fellow Hollywood stars for being tight-fisted, accusing them of refusing to give to charity despite having millions in the bank. She said she held a fundraiser at her home a few years ago, in which 40 showbiz guests were invited to donate cash to landmine victims - and came up with just $4,000 between them.

There was a time, she says, when she did not really understand celebrity. "It was because I didn't have any use for it. Now I can do something with it. It has become a means to an end, because, being public, I can go to Darfur and call a press conference and I can discuss what's going on there with the media, and it's picked up by Reuters and the BBC and around the world. I can also goto an area and fix a well or build a school because of this business I'm in. That makes sense in my mind.

"At the same time it can be very frustrating to come back from an area where you feel there's so much real life going on and suddenly people are just obsessing about what you're wearing or who you're dating. That's all fun, I guess, but shouldn't we also be talking about the serious problems facing the world? There really is so much good we can all do."

She cites Alexander as a pertinent film in terms of its relevance to the world today. "Hopefully, if we do films like this, people will realise that there is more to what has been and is going on in the world, but people pick up on what they want to pick up on. I think that Alexander, as a leader, was a complex person. There were some very negative things about him, what he did and why he did it, and I think that's represented in the film. But I do think there are so many good things about him, too, and the way he believed in what he was doing, the way he was not just the first person to say he was going to war but that he was also the first person to lead his troops into battle."

She is unstinting in her praise for her collaborators on the film. "I saw so many people working their asses off and having sleepless nights and caring so much just to get everything right. It exhausted me just to watch all the effort Colin and Oliver were putting into the film. I think they're both great men. They're so open, and they never hold back anything, so I could trust them at all times. They are what they are - and I had a lot of fun working with them, too."

Going from Alexander to co-starring with Pitt in Mr And Mrs Smith was "a very, very different experience", she says, "because it's a film about marriage, and we're partners in it, so we formed just a great friendship while we were making it. Alexander is a much heavier, darker piece, whereas Mr And Mrs Smith is more of a fun movie."

Pitt is "lovely to work with", she says, and she seems bemused by the speculation that they are an item, which was fuelled again recently when Pitt's wife, Jennifer Aniston, was observed not wearing her wedding ring at an airport. "My view on this," Jolie says, "is that I believe most people know what gossip is and they don't take it seriously.

"I actually think it's funny that there's so much gossip about me. If it were true I would be leading such a crazy, hectic life, dating all these men I'm supposed to be involved with. I don't know where I'd get the time to date all of them. It seems like I've been connected to every single actor who's alive. Then again, if it was just one it might be a dangerous rumour, but there have been so many actors linked to me that I just think it's laughable, and I'm sure most other people think the same.

"If I was ever to get involved in a serious relationship with another man the most important thing for me is that he would have to be a great father. I haven't found that guy. Not yet, anyhow, but I'm not expecting that any time soon, and I'm not counting on that to raise my child. The men in my life right now, I don't mix with my son in any way. Not that I am that active in my life. But I have occasional evenings."

Alexander is released tomorrow