Why Kenneth Branagh? In lining up Russell Crowe for a long-planned Hollywood biopic that has yet to be made, the German director Wolfgang Petersen announced that there was no Englishman of sufficient stature to play Sir Ernest Shackleton. There may not be at the global box office. Branagh's own recent form on the big screen, culiminating in his unloved 'Love's Labour's Lost', has seen him long since turfed off the A-list. But he does seem to be the perfect fit for Shackleton. It's not just age, or the physical coincidence of a squat burliness and a square jaw. Branagh, 40, is also the same character type. He has a long history as an actor-manager whom the public could never quite bring themselves to love: that's Sir Ernest all over. Irish-born, both men give off an aura of not quite fitting in, of channelling their ferocious energy into going it alone and flying by the seat of their pants. You imagine that the actor would be as little fun to live with as the explorer.

"What appealed to me was his heroism," says Branagh. "Charles Sturridge has done a convincingly complex picture of a fellow who was much flawed, a bit reckless with money, ruthless in some ways, but in charge the best was brought out of him. His imagination and vision about the care of others, and genuine interest in the ways groups work, is something that has fascinated me. Acting and directing gives me some experience of the Shackletonian technique of never asking people to do anything you wouldn't be doing yourself. All of those men saw Shackleton do everything they were doing and, at the end of the day, make sure everybody else had eaten first, check on their welfare, make a plan for the next day, which he had to be open enough to share with other people and have criticised at a point when his own ego would be enormously fragile, and then get up and do it all the next day. The spiritual, mental and physical capacity for going the extra mile was something that kept their belief in him alive."

This wasn't the first time someone had made the connection. A dozen years ago, Branagh was approached by an American film maker with a script that was not good enough to lure him. The writer-director Charles Sturridge says he was the only person he could think of to play the role. "I knew for certain that there had to be great physicality. But his ability to persuade was completely crucial to how Shackleton operated. Very early on, I read a description of an American woman say, 'Only Churchill have I ever seen compare with him in his ability to speak in public'."

On both stage and screen, Branagh made his name as another great English orator, Shakespeare's 'Henry V'. When Sturridge offered the role over a Chinses meal in Knightsbridge last year, not a word had been written. Branagh accepted before they ordered. He had never played a nonfictional character. Already a Scott enthusiast - "because he writes so beautifully and so evocatively" - he began immersing himself in polar literature. He found in Shackleton's diary "this unusual possibility to check almost day by day across the expedition. You to some extent begin to feel that you uniquely have a chance to get closer to the man than you might otherwise do."

In Dundee, Branagh visited the Discovery, the ship for Scott's 1901 expedition, which gave Shackleton his first taste of the south. He looked in on the James Caird in Dulwich College, an exact replica of which was built for the drama. He listened to gramaophone recordings of Shackleton's voice. "People talk about him having some Irish lilt, but in the undeniably self-conscious recordings of the time he wounded very English, with maybe a little intonation that had a Celtic influence. But it was very exciting to hear his voice." He met Shackleton's grand-daughter Alexandra, who brought to the cast read-through the cooking pot that made the crossing of South Georgia. "The preparatory process was enjoyable even beyond the excitement of playing the part."

Branagh also spoke to explorers. "One of the things that struck me about explorers, and Shackleton in particular, was that their drive to get to the start of the expedition finally outweighed any considerations about the logic of what they were doing. When I talk to modern explorers, you see them do whatever they can just to get the money and just to get to the start point, at the earliest opportunity - even if, as was definitely the case with Shackleton, they are ill prepared. 'I've got there so now nobody canstop me.' That sort of thing goes on with films." The polar explorer David Hempleman-Admas also gave him an insight into "the experience of 18 months' or two years' preparation for the expedition, and then absolutely ghastly challenge of the first half-hour, as it were when you leave Pinner on the way to the airport and a racing mind battles with the contradiction inherent in 'I've sacrificed family life and every spare hour I've had over the last two years, and I don't want to go now'."