Almost a year before production started on Michael Mann’s film, Ali, Will Smith began his preparation. Playing a real person is tough for any actor: playing one of the most recognisable men in the world must have seemed a hell of a challenge, even for a seasoned actor like Smith.

“I felt like we had a lot in common,” he says. “I understand his love of life. I understand his love of children. I understand his desire to have people and friends around. I can relate to a lot of things that drive him. But I don’t know if I will ever be able to achieve that kind of greatness in my life. Playing the role is really sweet. But knowing what he went through to become the greatest of all time, you know things had to taste a little bitter sometimes, too.”

When the Ali company was filming nights on 79th Street in Chicago in the dead of winter, thousands came out. And the young who were there to see Will Smith were outnumbered by the older men who remembered the days when Muhammad Ali and his pals used to hang out there. Later, when the film-makers needed a few thousand extras and invited the inhabitants of Maputo in Mozambique to come and “watch the filming” of the ‘Rumble in the Jungle’ sequences, 30,000 people turned up.

The phrase ‘living legend’ is frequently overused, but it could have been coined for Muhammad Ali. “He doesn’t have to open his mouth,” says executive producer Howard Bingham, who has known the Champ for nearly four decades. “When he walks into a room, he controls the room. He’s loved by everybody; by the world.”

Will Smith as Ali

All of which made Smith’s task daunting. There were two things he had to do from the outset: look like Ali, and talk like Ali. Then there was a third, which really was impossible: fight like Ali. In his heyday, which lasted from 1964 to 1978 (with a lot of time off in the middle when the WBA stripped him of his title), Ali was the best fighter in the world. In the opinion of some, he is the best fighter there ever was. And he is probably the only former fighter who can still command the respect of millions, as witness the moment in the summer of 1996, when he lit the flame to start the Atlanta Olympics.

When it came to talking like Ali, Smith’s powers of mimicry amazed even those familiar with the star’s career. After practising the accent of Louisville, Kentucky, where the boxer - then still called Cassius Clay - grew up in the forties and fifties, Smith took to calling up Lonnie Ali, the champ’s wife. “I could have sworn it was Muhammad 20 or 30 years ago,” she says. Smith even tried it out on Ali himself. “The first time Will met Ali, he started imitating him, right to his face,” recalls Bingham. “Will broke right into his Ali routine. He was really good, too. After a minute or so of watching him, Ali turned to me and asked, ‘Was I really that crazy?’”