Long kick to touch: Matt Damon, who was doubtful he could play Francois Pienaar in 'Invictus'

Actors are larger than life: that's the illusion of cinema. Knowing this didn't really reassure Matt Damon when Clint Eastwood rang up to ask him to portray Francois Pienaar, captain of the Springboks rugby team that helped unite South Africa by winning the 1995 rugby world cup soon after Nelson Mandela became president.


"Clint," Damon said, "Francois is huge and I'm an average-sized guy. People know what I look like and they know what he looks like. How are we going to get around this?" Eastwood just laughed. "Oh hell," he replied, "don't worry about that. Maybe you won't look 6'4" but maybe also people won't say you're 5'10". We'll just get people to not ask the question."


Damon only knew of rugby as a game played by some of his Harvard room-mates: "They always seemed to break a nose or an arm." He spent six months in a gym hoping to build up enough muscle to seem plausible as the iconic Afrikaaner wing-forward who at Mandela's instigation turned a sport that symbolised apartheid into an instrument for racial reconciliation. "There were all sorts of little tricks Clint played, like putting the camera higher and framing me to look a little larger, or wearing a little insole in my shoe to give me another inch of height."


Invictus (the word means 'unconquered') transcends the cliché conventions of a Hollywood biopic by focusing on the extraordinary relationship between these two men – one just released after 27 years locked up on Robben Island, the other part of the white elite that had put him there – and treating it as an emotional device to dramatise Mandela's dream of a 'rainbow nation' in simple but powerful human terms. Some might argue that politics doesn't belong in sport, but Invictus shows how the mix can become a powerful force for reconciliation.


The idea goes back to a question put to Mandela years ago at the publication of his book Long Walk To Freedom, when he was asked if it became a movie, who would he like to portray him. "Morgan Freeman," replied Mandela. "So it was on the cards for me," says Freeman. "Sooner or later I was going to do it. I don't think you can sum Mandela up, but I think in the film we tap a lot of what he is."


Mandela seems to think so too. He watched a screening with Freeman. "He smiled a lot and nodded," says Freeman. "When I first came on screen he leaned over to me and said, 'I know this fellow.' I got the impression that he wasn't embarrassed."


Freeman has been observing Mandela for years. "As soon as I got the notion that one of these days I would be playing him it became a question of just paying close attention to him. But it's not like I'd call him up in the middle of the night and say, 'Yo, Mandiba, what's going on?'"


It was much the same with Damon. When he first got to South Africa, Pienaar invited him to dinner at his house. "He opened the door and I looked up at him and said, 'I look much bigger on the screen.' He laughed and gave me a hug, and that was it. We were off and running.'"


Damon, who is married to Argentine-born Luciana Barroso, met Mandela soon after. "I was asked to bring our kids Alexia, Luciana and Gia, which was a real thrill. I spent the time just watching him bounce our babies on his knee. He was just absolutely wonderful with them. We have wonderful photographs, so they'll grow up knowing who he is."


Eastwood's idea of realism was to employ real rugby players. "So a lot of the stuff we shot was what we call free play, which is like letting these guys go and nail each other, and for Clint to catch that," said Damon. "His biggest challenge was to stay out of the way."


The footage of Damon being rucked by burly All Black forwards as he lies in the Ellis Park mud looks painfully convincing. Did he pick up any injuries? "Hell no, the biggest reason being that it was my stunt double that was in there most of the time. But look, any time you're making a movie, it's all choreography."


His irreverent humour perhaps explains why he and Ben Affleck are such close friends. They grew up near each other in Cambridge, Massachussetts: his mother moved there when he was two following her divorce from his father in 1972. After dropping out of university and heading off to Hollywood to become an actor – he had a bit part in Mystic Pizza before playing an opium-addicted soldier in Courage Under Fire – he co-authored Good Will Hunting with Affleck, an Academy Award-winning original screenplay about a young maths genius whose talent is encouraged by Robin Williams. It grossed over $100m and earned him a $500,000 pay cheque. His involvement in two major Hollywood franchises – playing an assassin with no memory in The Bourne Supremacy and a cocky young thief hired by George Clooney in Oceans Eleven – now allows him take on politically challenging roles, whether as an espionage analyst in Syriana, a rogue CIA agent in The Good Shepherd or an undercover mobster in Martin Scorsese's The Departed.


Coming from a liberal family – his father was a stockbroker, his mother an education professor at Lesley University – he was prominent in student protests, wearing black Free Nelson Mandela ribbons at high school. He remembers Nelson Mandela visiting Boston when he was at Harvard. "It was a very, very big deal," he says.


Revenge and retribution are recurring themes in Eastwood's films. "I guess without thinking about it too deeply I am interested in the results of violence and the effect of it on the perpetrators as well as the victims as well." Yet in Invictus he celebrates the importance of turning the other cheek. "Are you going soft?" Damon teases him. "I could never have been Mandela," he admits. "Dirty Harry or Josey Wales would never be forgiving in that way. I think politicians around the world could learn from his creativity in bringing people together."


When Invictus premiered in Los Angeles before Christmas, Francois Pienaar was so moved by a key scene in which Damon re-enacts his visit to the tiny cell on Robben Island where Mandela spent so much of his life that his two sons said, "Dad, are you alright? You're crying."


Last Tuesday, Freeman won a best actor Oscar nomination and Damon got the nod for best supporting actor. "Awards, you know, are just pats on the back," says Freeman. "They're really the only reason to make movies," Damon adds, deadpan. "I know I speak for all of us when I say that." Two-time winner Eastwood, surprisingly overlooked by the Academy voters, just shrugs. "You don't win awards, you're given them. It's not like a person at the swimming meet or a track meet, where you actually have to win a race."


Eastwood, who will be 80 this year, is already in preproduction for Hereafter, a Peter Morgan supernatural thriller starring Damon.


"I enjoy working again with people I know," he says. "I never planned to still be working, but I just enjoy it. I'm at an age when I can take on more challenges than I have in the past because I know more. So I figure I'll just continue until somebody hits me over the head and says get out."