Before Matt Damon could even think about working on the accent and muscles needed to portray South African rugby star Francois Pienaar in Invictus, he had a much more pressing concern: his height.

“The very first day I got to South Africa, Francois invited me over to his house for a gourmet dinner that he was cooking,” Damon remembers about meeting the six-foot-three-inch Pienaar. “He opened the door and I looked up at him, and the first thing I ever said to Francois Pienaar in my life was, ‘I look much bigger on film.’”

Thou-gh Damon is no stra-nger to heavy training after his work in the Bourne films, that was nothing compared to Invictus, he says.

“I was in better shape for this movie,” Damon insists, owing the extra effort to his budding friendship with Pienaar. “This is his life. I don’t want to embarrass him. If Jason Bourne looks a little flabby, that’s on me. This is the fictionalization of somebody’s actual life. I didn’t want to let him down.”

As an added challenge, his work on Invictus came right after this fall’s The Informant, in which he packed on a considerable amount of flab. “I had a good time putting the weight on, and then a tough time reshaping the weight,” he says.

Though physically daunting, Damon was committed to doing the film because of the story it tells: Nelson Mandela using the 1995 Rugby World Cup to unite the citizens of South Africa after apartheid.

“As actors, we react to the material that’s out there. I probably just react more strongly to things that I feel have some social value,” he says.

“I remember (Mandela’s) Boston visit. I remember the whole world tour. In my high school, we had the ‘Free Nelson Mandela’ ribbons. Kids were wearing those before they knew who he was.”

Damon credits director Clint Eastwood for an incredibly easy working experience.

“You walk on some movie sets and it’s like walking into an emergency room. And that tension bleeds into the performances and the film itself,” he says. “But Clint just leads an incredibly tight ship.”

Damon was particularly impressed with Eastwood’s commitment to keeping things moving.

“After you do a take, he goes, ‘Well, let’s not f--- this up by thinking about it too much.’”

• Invictus opens in theatres Friday