Gerard Butler is a very big and very muscular, and can actually act (yes, I know about "300", but that's not his only film).

I think there's a distinction between Truman Capote/Philip Seymour Hoffman and Pienaar/Damon. Hoffman captured all the aspects of Capote that were important: the ambition, the ruthlessness, his status as an outsider, his sexuality, his attraction to the murderer he wrote about. His physical size wasn't important.

One of the important things about Pienaar was how he was such a perfect embodiment of the Afrikaner rugby ideal - not only a great player, but huge, blond and very, very tough - in fact, the very embodiment of everything that most black South Africans hated about rugby. Rugby is a hard, violent (though beautiful IMHO) sport and in South Africa it was the white man's sport. For black South Africans the size and strength and toughness admired by white rugby fans was very closely linked to the arrogance and sense of superiority displayed by whites. I will remember to the day I die the moment when Nelson Mandela, wearing a Springbok jersey, presented the World Cup to Francois Pienaar, who towered over him (and Mandela is a big man himself). I think it will be difficult to recreate the drama of that moment, and of the entire World Cup, without an actor who can at least approximate Pienaar's physical presence.

It's only fair to point out that Pienaar may have looked like the platonic ideal of an Afrikaner rugby man, but he was far from being a racist. His leadership had a lot to do with the Springboks becoming the symbol of national unity that Mandela wanted them to be. I remember a TV interview around the time of the 1995 World Cup in which he admitted that the rest of the world had been right to boycott the Springboks.