Brad Pitt is fully aware of the irony of playing one half of a couple at war, he tells Steve Pratt. Especially when the other of the couple in Mr and Mrs Smith is Angelina Jolie, the actress who fuelled rumours about the reasons for the breakdown of his marriage to Jennifer Aniston.

WHEN you're one of Hollywood's top leading men whose marriage is on the rocks, the last thing you want is to star in a film about a married couple at war with each other. And then, when you're linked romantically with your leading lady, the result can only be a media scrum that's a publicist's dream but a personal nightmare.

It doesn't matter that Brad Pitt and Angelina Jolie deny they're involved with each other. Everybody loves to believe that two of the cinema's most beautiful people are an item.

They certainly look good as Mr And Mrs Smith, a movie described by the makers as "a sexy action adventure", which covers all bases. The couple are, unknown to each other, both killers for hire and their relationship hits trouble after they're assigned to kill each other.

Pitt has called the movie a metaphor for marriage. "That's what made me laugh in the beginning when I read the script. The fact that you had a husband and wife who literally wanted to kill each other," he explains. "I loved that. Of course, I signed on for this film about two years ago and I was in a different place."

What he means is that he was still with his wife, Friends star Jennifer Aniston, at that time. They're now getting a divorce.

Mr And Mrs Smith is a big, mainstream movie and Pitt is happy to do purely fun roles. "I definitely feel like I've been doing and getting more into the lighter, fluffier stuff. But there still has to be an undercurrent to the film and the script that's different or unexpected, or I'm not very happy. And if I'm not very happy, then I'm not a very nice person to be around," he says.

He doesn't see any subliminal lessons about love and relationships in the film. "I really try to stay away from lessons. This one reveals some truths and you can have a laugh in the process," he says.

"The ultimate truth in Mr And Mrs Smith is that it's very easy to lose yourself along the way and to lose the original idea of what brought two people together, that there's no manual that guides you past that initial coming together. Think of how many couples you know in your life that you look at and say, 'I want to have that'. And it's not what you always think."

As well as making movies, Pitt has become more publicly involved in issues, including fighting poverty in Africa and California's stem cell research legislation.

"A lot of people put a whole lot of work into the stem cell legislation before I came along, I got there just in time to help tip it over. I'm just starting to go mental with all the misinformation that is out there," he says.

"Forget how I'm represented, it drives me crazy when I see all the misinformation when it comes to politics and how it affects people's lives, so I feel I've got to get in there. Turning 40 did a lot for me. I've got no more excuses. At some point you've got to pick up your corner of the sheet and help out.

"When I look at what's going on in Africa, it doesn't even make our news. There's an entire generation of people there being wiped out and this thing cannot get any press. And me, I can't get out of the press and for what? Half of America is divorced. Maybe I can somehow use this to parlay it into another direction."

Tomb Raider star Jolie rivals her co-star for attention over her love life and looks. The pair got along fine on set with her describing Pitt as "very easy to work with, very professional and really good at all this kind of action stuff".

She may regard herself as "kind of goofy and I'm just a mum" but men have been known to be intimidated by her. "I've always been one of the boys growing up. It's probably easier to approach me as another guy because the girl side of me is very private. I'm not soft and emotional," she says.

"I hung out more with guys growing up. I never really had girlfriends, I spent more time with guys. But now, as I'm older and I have a child, I've found the friendship of other women."

It's not a matter of balancing her roles as mom and actress. She has an adopted Cambodian son, Maddox, and would like to adopt more babies. "I don't even see it as a balance. I feel like I'm Mad's mum and that's all I am. I have a job, and it's a good job, and I like being an actor and I'm grateful for the job, but it's not what I am," she says.

Her role as a United Nations Goodwill Ambassador, which she took up four years ago, is occupying a bigger part of her life. "When I first started working I was very focused on just visiting refugee camps, learning and spreading awareness. Now I've been more aware of formulations, different conventions and treaties. I'm pushing a Bill in Congress. I've just become more active, not meaning to but it just happened.

"I would actually love to do it as a full-time job but I know that it's a means to an end. Acting's given me the financial ability to build a lot of things and do a lot of good things, and gives me the option to speak to the press and bring attention. So it's not a hard job."

After completing Mr And Mrs Smith, she began filming a documentary, which she's financing herself, about the plight of refugees around the world.

She's equally willing to talk about sex and sexuality. "Sex is healthy and it's the reason I was outspoken about having lovers," she says. "There are a lot of single women that have an issue with it, they keep accidentally bringing men home and confusing their children. They don't know how to make that division and to satisfy themselves as women, as sexual beings.

"I went a year and a half just being a mum and not being near another man, not going to dinner, not having sex. I felt I wanted to feel like a woman but didn't want to confuse my son."

* Mr And Mrs Smith (15) opens in cinemas today.

Published: 09/06/2005