Brad Pitt is musing on the problems inherent with the fame that pervades his daily life. He and Angelina Jolie cannot go out in public without being mobbed by fans and photographers, and outside the front of the Toronto hotel where he and I are talking, hordes of paparazzi have gathered, although they won't even get a glimpse of their targets.

"I haven't seen a hotel lobby in 15 years; I have to go in the back door and out the same way," Pitt says. "I've been in this city for 48 hours and the only time I've been out was when I walked across the street to the premiere of the movie."

But, he says cheerfully, there is an upside to being so famous: "It's a trade-off and we get our moments."

One of them, he recalls, happened the previous evening at the premiere of his latest film, Moneyball, when he and Jolie were on the red carpet and hundreds of fans pushing against the barriers were screaming their names.

"There was a woman in line who was crying and I went over to her and asked if she was OK because I thought she was getting crushed from behind, which is what usually happens," he says. "She couldn't speak and her friend said: 'She loves Angie. She wants to meet Angie.' So she got to meet Angie and that was something she won't forget. It was a special moment for her and there's something very lovely about that."

Ever since he removed his shirt for Geena Davis 20 years ago as a bad boy in his breakthrough film, Thelma & Louise, Pitt, now 47, has been a movie star and sex symbol, although it is not something he thinks about or wants to trade on. In fact, he has been resisting glamour and instead choosing roles that are anything but sexy. In The Curious Case of Benjamin Button he played a man born in his 80s who aged backwards; in The Tree of Life he was a gruff, surly, 1950s father; and in Moneyball he plays Billy Beane, the fiercely competitive, tobacco-chewing, chair-throwing general manager of the Oakland Athletics US baseball team.

It is the first time Pitt has played a real-life person, and although not a big baseball fan himself, the project is one he felt so passionately about that he became its producer, working for four years to bring it to the screen, shepherding it through several script drafts and dealing with the turmoil caused by actors and directors who came and went.

Based on the book Moneyball: The Art of Winning an Unfair Game by Michael Lewis, the film tells how a decade ago Beane took the lowly A's to Major League success by treating baseball as a science and choosing players based on their statistics rather than on scouts' recommendations. With the help of a statistics wizard he hired, he assembled a winning team without paying star salaries.

"I became obsessed with the book and what the guys in Oakland did in re-imagining the game - like questioning religion in a way," Pitt says. "It's about how we value things: how we value each other, how we value ourselves and how we decide who's a winner based on those values. The film questions the very idea of how to define success. The players these guys hired thought of themselves as failures and suddenly they were given the opportunity to play, proved worthy and changed the game a bit."

Pitt is amiable as he relaxes in a straight-backed chair with a cup of coffee on the table beside him. His blond hair is long and he has several days' stubble on his face; he wears tinted prescription glasses and a black, long-sleeved sweatshirt and beige slacks. He still has a slight scar under his left eye, a reminder from when he was hit by a baseball as a schoolboy and needed 18 stitches.

Pitt is one of the few established stars today who has no interest whatsoever in his public profile and is embarrassed by the good looks bestowed on him by genetic good fortune.

"He's a very secure person," says Steven Soderbergh, who directed him in Ocean's Thirteen and was originally due to direct Moneyball but gave way to Bennett Miller (Capote) after a disagreement with Sony over the budget. "He would be exactly the same person whether stardom had happened to him or not. He is just who he is, one of the coolest people on the planet."

Pitt grew up in Springfield, Missouri and attended the University of Missouri, which he left two credits short of a degree in journalism. He had set his sights on a career in advertising but instead succumbed to the lure of Hollywood.

"I didn't really think very far ahead other than the realisation that I loved films and I wanted to give it a shot," he recalls.

So, telling his parents he was going to attend an art college in Pasadena, he loaded up his car and headed west to California. He studied acting while working at odd jobs, including driving strip-a-gram girls around the city, dressing up in a chicken costume to promote a fast-food restaurant and delivering refrigerators.

He landed small roles in daytime soap operas, appeared in three episodes of Dallas and entered feature films through the familiar road of supporting roles in slasher films and teen sex comedies. His breakthrough came as the hitchhiking charmer who seduces Davis and then steals her money in the 1991 film Thelma & Louise.

Instead of taking the Tom Cruise route and cashing in on his instant stardom and good looks, Pitt opted for smaller, more varied roles that offered personal satisfaction rather than career momentum. He played the troubled younger brother in Robert Redford's A River Runs Through It, the stoner roommate in the Quentin Tarantino-scripted True Romance, an arrogant cop in Seven and the twitching mental patient in 12 Monkeys. He co-starred twice with Anthony Hopkins, in Legends of the Fall and Meet Joe Black, and played an Irish gypsy in Guy Ritchie's Snatch.

Teaming up with his friend George Clooney, he co-starred in Ocean's Eleven, Twelve and Thirteen and two years ago reunited with Tarantino for Inglourious Basterds. He has been nominated for Oscars twice, once for his supporting role in 12 Monkeys and the second time as Best Actor in The Curious Case of Benjamin Button.

His romantic life, too, was active, and in the late 1980s and early 1990s he was involved in a series of relationships with several of his co-stars, including Robin Givens, Jill Schoelen, Juliette Lewis and Gwyneth Paltrow, whom he dated from 1994 to 1997.

He met Jolie when they were cast in the action thriller Mr and Mrs Smith, but they both maintain their romance began only after his four-year marriage to Jennifer Aniston ended. Ever since they were first spotted in public together in the spring of 2005, Brangelina, as they were dubbed, have been the subject of wild speculation and rumours about their relationship. They are regularly reported to be either separating, getting married or having more children - all reports they ignore.

As is well-known, they have three foreign-born adopted children (Maddox, 10, from Cambodia; Pax, 7, from Vietnam; and Zahara, 6, from Ethiopia) and three biological children (Shiloh, 5, and 3-year-old-twins Vivienne and Knox). The children remain at home with their nannies during the three days Pitt and Jolie are in Toronto but, says Pitt, Maddox is keeping in regular touch via his new mobile phone.

"I'm enjoying life more than ever since the family's come along and now I always consider whether it's worth investing time in any project that's going to take me away from the family," says Pitt, who recently caused quite a stir by saying he plans to retire as an actor in three years (he quickly downplayed the comment). "My concerns are all with family and their safety. Am I spending enough time with them and am I giving the opportunity and instructions that are important to them? Am I getting that right? Once family comes along it becomes a matter of life and death. It's chaos at times but there's such joy in the house. I've never felt more enriched and I've become a better person because of them."

The question of marriage is a recurring one and he says: "Angie and I talk about it and we've already made a bigger commitment and that's raising a family together, so everything else is secondary."

Pitt is filming the zombie tale World War Z while Jolie is about to embark on promotional duties for In the Land of Blood and Honey, her directorial debut that she also wrote. But while filming keeps them busy, charity work takes up even more time.

While he shuns any limelight on his personal life and is embarrassed and slightly horrified by the "Sexiest Man Alive" title that People magazine twice has bestowed upon him, Pitt, like Jolie, has learnt to make good use of his celebrity status to promote the causes close to their hearts, such as fighting poverty and hunger.

"Of course it helps because there's an automatic spotlight on us and it's hard to get out of that spotlight," he says. "It occurred to me a few years ago that there were so many people in need and so many stories that needed to get out that maybe I could be a conduit in that way."

His and Jolie's filming schedules and charity work take them around the world, and at first the children went to school in whichever country they happened to be at the time but now they are being schooled at home. When they are not away on film locations or on charity missions, Pitt and Jolie divide their time between homes in Los Angeles and a 1,000-acre estate near Aix-en-Provence in the south of France, where they are trying to spend more time.

"It's really nice for the kids and they can have more of a normal life there where people are kind and don't get in their faces and the photographers are a little less aggressive and rabid," he says.

There is no doubt as to his top priority.

"What's valuable to me has become clearer as I've got older," he says. "To me, it's about the value of your time and your day and the value of the people you spend it with. It's about me being a strong father and guide and a good match for my significant other. Then if I'm going to go to work it must be something of value to me.

"I still value the work I do but at the same time I value getting home to the kids more."