Oscar-winning director Bille August has brought the memoirs of one of Nelson Mandela's jailers to the screen, with Dennis Haysbert playing the famous prisoner -- a part that he said regularly left him in tears.

Goodbye Bafana, which stars Joseph Fiennes as prison guard James Gregory, premiered on Sunday at the Berlin Film Festival, where it is competing for the Golden Bear award.

Gregory, who said he built up a friendship with the former South African president over 17 years, worked at the Robben Island prison in the office that handled prisoners's mail and sat in on visits.

"The word is daunting, I would use intimidating, to play a man whose love for his country outweighed his love for himself, his youth, his family," Haysbert, who previously played a fictional black United States president in Fox's TV series 24, said at a news conference.

"The sacrifices he made were profoundly sad to me," Haysbert said. "The making of the movie for me was very hard ... every night, I went home, I would have a glass of wine and just cry."

Fiennes said that a key challenge in acting a white South African who at first supported apartheid was to understand the strength of "a certain conditioning which is pretty scary and abhorrent at times".

August, a Dane who won an Oscar for best foreign film in 1989 for Pelle the Conqueror and also directed The House Of The Spirits, found that telling Mandela's story from Gregory's viewpoint was compelling.

"What I think is so great about this film is that it is seen from the opponent's point of view, which makes Mandela's views ... even more right," he said.

As well as Gregory's book, also titled Goodbye Bafana, he said the filmmakers talked to ex-prisoners, ex-wardens and "all possible sources we could".

Mandela spent 27 years as a prisoner of apartheid, most of it at hard labour at the maximum-security prison on Robben Island. He was released from prison in 1990 and led the negotiations that led to South Africa's first all-race election in 1994. - Sapa-AP