He's one of the biggest stars on the planet but Brad Pitt has announced he's going to retire in three years at the age of 50. As he stars in baseball flick Moneyball, he talks to Susan Griffin about longevity, losing streaks and bad choices.

When you say you're off to see Brad Pitt in person there's one request that will inevitably be made by men and women alike: "Tell me what he looks like in the flesh.

That's because Pitt is so famous he's almost taken on an otherworldly status.

The truth is the actor, voted 'sexiest man alive' countless times during his career, doesn't disappoint when he casually strides out onto the terrace of a Mexican hotel for a photocall to promote his new baseball movie Moneyball.

He's tall (a rarity among Hollywood's elite) and broad, having spent the last few months in training for the film World War Z, a post-apocalyptic horror, which has seen Pitt, his partner Angelina Jolie and their brood of six reside in Glasgow, London and Cornwall during filming.

He may be an A-lister, but he's rarely spotted on the red carpet these days, and while he smiles for the assorted cameras and cracks jokes with co-star Jonah Hill, his eyes remain hidden behind aviator sunglasses and his hands stay tucked into the pockets of cream linen trousers.

Moneyball is based on the controversial 2003 book Moneyball: The Art Of Winning An Unfair Game in which the author Michael Lewis wrote about the exploits of Billy Beane (Pitt), a former baseball player turned general manager of Oakland A's baseball team.

Beane revolutionised the way the baseball industry assessed its players by employing a statistical analysis that showed the qualities historically used to value players were outdated. He then bought players who'd been ignored by larger, more lucrative teams, but who would help his team to victory.

"Billy Beane's team had a $40 million payroll and they were trying to compete with teams with $240 million payrolls," says Pitt. "It forced these guys to back up and say: 'We can't fight how the other guys fight. We have to search for new baseball knowledge. We have to re-examine the sport and where we place value'."

In the 20 years since his big break with a 15-minute scene-stealing performance in Thelma & Louise, Pitt's managed to keep the audience guessing his next move.

He's depicted death in Meet Joe Black, an incoherent Irish brawler in Snatch, a Nazi killer in Inglourious Basterds and aged backwards in The Curious Case Of Benjamin Button.

"Film-making is about longevity," says Pitt. "Is it a quality picture? Is it a quality story? Is there something original about it?" These are the questions Pitt asks himself before embarking on a project today but admits he may have lost focus for a while and the result was The Mexican (2001), Spy Game (2001), Troy (2004) and Mr & Mrs Smith (2005).

The latter remains a personal favourite though, as it was the film on which he met Jolie, with whom he has adopted children Maddox, 10, Pax, seven, and Zahara, six, and biological children Shiloh, five, and twins Knox and Vivienne, three.

Pitt recently announced he's looking to retire at about 50, and as the producer of 20 titles perhaps he'll spend the autumn years of life behind the scenes.

"It's just about getting stories across that may have a more difficult time seeing the light of day," he says.

"That's all I want, to see movies get made that I believe should be made. And if my name, whatever it may or may not be worth, can help that process, then that's what I'm going to do."