Dennis Haysbert: The '24' actor lands the role of his life playing Nelson Mandela

"I'm a little jet-lagged," warns Dennis Haysbert, dropping his trim 6ft 4in frame into a chintz armchair in a London hotel. "If I doze off, just yell at me."

The 52-year-old has made a career out of playing characters whom you are convinced would never fall asleep on the job. In America, he is the face of Allstate insurance and the voice of the Military Channel. He is the actor the producers of 24 went to when they wanted a president with a strong moral backbone, and is currently playing the reliable-under-pressure field commander of a counter-terrorist team in David Mamet's hit series The Unit. And when the film-maker Bille August needed someone to play Nelson Mandela, in an adaptation of James Gregory's book, Goodbye Bafana: Nelson Mandela, My Prisoner, My Friend, he contacted Haysbert.

August's "beautiful letter" unsettled the actor. "I said, 'Boy, if I do this, what if I fail? What if I don't do it justice? If I mess this up, people are going to laugh. And what if [Mandela] gets pissed off?' I couldn't stand to disappoint one of the people I regard as one of the top five human beings ever to grace this planet," Haysbert says.

Told from the point of view of Gregory (Joseph Fiennes), who was a guard at the Robben Island penal colony, Goodbye Bafana is straitjacketed by an unstinting reverence for Mandela. But Haysbert performs well in a role that often demands he internalise his emotions. Mandela's tear ducts had been damaged by wind-blown grit when he was breaking rocks in a limestone quarry, making him physically unable to cry. Haysbert therefore had to let rip off set.

"Every time I went back to the hotel after a difficult scene or day, I'd have a glass or two of wine and just have a good cry. I'd darken the room and just wail."

The actor may never know what Mandela thinks of his performance. Mandela has never confirmed Gregory's claim that they became friends and there were reports that he once considered suing the former jailer. Mandela's friend and official biographer, Anthony Sampson, says Gregory rarely had contact with Mandela and fabricated the friendship from details he gleaned from letters he read as a censor. When I ask Haysbert about the controversy, he appears little troubled by it.

"I've heard a lot of those rumblings," he says dismissively. "But consider that this is the first film that actually got made, and the agendas of those who have been trying to get a Mandela film [made, but haven't succeeded]," he says. "And consider that even if it was through the letters, on Gregory's account it could be construed as a friendship because he is engaging this man in conversation about Apartheid, about the Freedom Charter, and so on. Even if it's a delusion, you're going to call him my friend. Either way, it makes it real for him. And maybe Mandela did use him. He was, after all, fighting a war."

Haysbert's most famous and popular role to date is still President David Palmer. So admired was Palmer, he says, that some commentators in America have written that one of the reasons why Barack Obama now has a chance of being elected to the White House is "because there was a good fictionalised President on television, whom everybody loved in every walk of life."

Filming President Palmer's assassination at the beginning of the fifth season was a deeply unpleasant experience for Haysbert - he wanted the producers to change their mind about killing off the character. But he was not being driven by ego or money, he says. "We have a legacy of killing off our leaders," says Haysbert. "Malcolm X, Dr Martin Luther King, JFK, RFK... Why do that to a fictional character that everybody loved, just for ratings?"

When he refused to go back and shoot the scene, one of his best friends from the show, Howard Gordon, was sent to convince him. Was he contractually obliged to return? "No, no, no," Haysbert says. "They had to come to me and I had to say yes." He lets out a long sigh.

When did he start regretting his decision? "I was regretting it while I was doing it," he says. "If I had to do it over again, I wouldn't do it. But then again I don't know what it would have done for The Unit, which is also a Fox studio show, if I hadn't gone back. So who knows?"

Since Haysbert's departure, 24 has found itself increasingly under fire over its violence. Kiefer Sutherland's Jack Bauer now uses torture almost as a matter of course to extract information. Haysbert has heard that Ann Coulter - the conservative pundit - and the talk radio host Rush Limbaugh have visited the set, while Vice-President Dick Cheney and the former Defense Secretary Donald Rumsfeld are said to be fans of the series. "Once it got to the torture, I think the show really defeated its own purpose," says Haysbert, "and it has been skewed malignantly in another direction. It's really, really sad that it happened."

The show's Republican-voting creator, Joel Surnow, recently said that 24 "makes people look at what we're dealing with" in terms of security threats. "There are not a lot of measures short of extreme measures that will get it done; America wants the war on terror fought by Jack Bauer. He's a patriot."

But Haysbert says the show is not reflecting the world we live in: "It's exploiting it."

Such has been the concern that, last November, Army Brigadier General Patrick Finnegan, the dean of the United States Military Academy at West Point, and three of the most experienced FBI and military interrogators in America visited the 24 producers and writers to warn them that many cadets now regard torture as a legitimate practice because of what they have seen on the show.

"When you start citing a show," says Haysbert, "when cadets from West Point are starting to say, 'Well, 24 does it, Jack Bauer does it, why can't we?' you've got to say something."

'Goodbye Bafana' opens on Friday; 'The Unit' screens on Tuesdays on Bravo at 10pm