Clint Eastwood didn't know much about Nelson Mandela before 'Invictus'.

The Hollywood veteran, who directed Morgan Freeman and Matt Damon in the tale of the first black South African president teaming up with the captain of the Springbok's rugby team to create racial harmony, admits it wasn't until he visited Mandela's prison cell that he truly understood him.

He said: "It was when I visited his former cell at Robben Island that I realised what calibre this man is. A little cell without a toilet and at daylight hammering stones or working in the mines.

"And what does this man do after nearly 28 years in jail? He comes out and talks about forgiveness. He is without a doubt one of the most incredible people on this earth."

Discussing why he chose to helm the project, Clint told "I always try to find themes which could interest a lot of people. Sometimes you read a book and decide to make it into a film.

"Sometimes the things come to you. In this case Morgan Freeman came to me and said, 'I've got a great book. I'll send it to you. You should make a movie out of it.' "