Dennis Haysbert isn't really a world leader - he just plays them in movies and on TV. Donald Clarke talks to the monolithic actor about David Palmer and Nelson Mandela

JUST before I make my way in to meet Dennis Haysbert I have a worrying conversation with a colleague from another media outlet. It seems that the actor, famous for playing the American president in 24, was so overcome with jet lag that he actually fell asleep during his interview.

Haysbert, a charismatic African-American with a warm, comforting voice, never comes across as the most animated of characters on screen, so it chills the blood to think what effects extreme exhaustion might have on his conversation.

Well, at no stage in our discussion would you mistake Dennis for Quentin Tarantino, but he manages to stumble his way through entire sentences and just about maintains consciousness throughout. It helps that he is promoting a project about which he cares very much. Bille August's Goodbye Bafana, a solid if unadventurous piece of work, details the relationship between Nelson Mandela and James Gregory, one of the great man's warders in prison. Joseph Fiennes plays the jailer. Haysbert plays the ANC leader.

"When Bille asked me, I seriously thought about saying no," Haysbert drawls. "The fact that he [ Mandela] is still alive worried me. I immediately wondered if I was up to the task. But then I thought, I really love the man. What he did is so important. This man sacrificed everything: his youth, his child, the raising of his other children, his marriage, the lives of his friends. All this to realise a dream of a united South Africa. That is a story worth telling."

Haysbert, born in the suburbs of San Francisco 52 years ago, will, if prompted, explain how he regards Mandela as one of the five most significant figures in recent history. It is certainly true that few other international leaders - and certainly none with a revolutionary past - have managed to maintain such levels of affection in the world beyond their own countries.

"He went into prison as a criminal and he came out a saint," Haysbert says. "And actually, in a sense, the guards themselves were in prison and he freed them too. Somebody once asked him the question: 'What was it inside you that helped you spend 27 years in prison?' He said: 'I had to free my jailers.' And that is what he did."

Now, here we run into a spot of bother. Goodbye Bafana, which was largely filmed on the actual locations in South Africa, suggests that Gregory, touchingly played by Fiennes, came to be a close friend of Mandela and that the jailer, originally an archetypal racist, softened as a result of his conversations with the detainee.

Unhappily for the film-makers, the late Anthony Sampson, Mandela's official biographer, has written that Gregory barely ever spoke to his prisoner and that he fabricated the account, originally published as a book, merely as a way of making money. Gregory is now dead and Mandela has remained ominously silent on the matter.

"That doesn't lesson the impact of the film," Haysbert says cautiously. "We didn't take the book as necessarily being a verbatim account. And I think it is as much about Dennis Haysbert accepting Joseph Fiennes as it is about those characters. Maybe the film does romanticise the relationship, but I think it still works."

I take this to mean we can regard the film's Gregory as an artful variation on the real thing. Fair enough, but one is still bound to ask whether Dennis feels that Gregory did have a close relationship with Mandela.

"Oh, I'm sure he did," he says. "Maybe it wasn't as close in real life as it is in the film, but you could not help but be affected by Nelson. You've seen the man speak. How could you not be moved by him?"

The controversy gathering around Goodbye Bafana will do no harm to Haysbert's growing reputation as a hugely likeable character actor. He has worked fairly consistently since he stumbled into acting 30 years ago - that's him as the doomed driver in Michael Mann's Heat and the ballplayer addicted to voodoo in Major League - but it is only recently that he has achieved proper fame. I wonder if he regrets not becoming a movie star in his 20s when he could have really abused his prominence.

"Everything in its own time," he laughs. "There are no accidents. I may not have been ready at that time. Right now I can handle the success."

What really changed things was his performance as David Palmer - first senator, then president - in the breathless television series 24. Apparently a man of strong moral beliefs, Haysbert finally wakes up properly when I mention that earlier interviews suggest he was unhappy about his character being killed off in season five. I am pretty sure that his outrage does not stem merely from his losing a good gig.

"It was just a stunt for ratings," he says. "We created the first ever African- American president. That is important. Don't kill him off. Let him ride into the sunset. Our legacy is that all our leaders have been killed: Martin Luther King, Malcolm X, Bobby Kennedy. Let's do something different for once. Let's break the pattern. Why not have him confront evil in the last episode?"

My goodness. He's really on a roll here.

"Do anything else. But by killing him you are feeding a legacy of our country that makes us weak. Why would you do that? Why? Why?"

I expect the next person interviewing Dennis Haysbert to thank me for properly waking him up.

Goodbye Bafana opens today