The actor was intrigued.

The camera is set up on an ice floe engulfed in freezing fog off the coast of Greenland. A wind machine blasts a polar blizzard across floating pack ice, which cracks and heaves. Leaning into the storm at 45 degrees,Kenneth Branagh begins to drag a heavy sledge laboriously over the ice. False snow made from potato flakes cakes his eyebrows and beard, but it is real ice that stings his face like fish-hooks, real freezing water that saturates his skimpy Edwardian explorer's costume. Branagh has trained for this by hauling a tractor tyre weighted with rocks over Chobham Common, near his home in Surrey. "At the front of my mind was the realisation that the heroic figure I was portraying had actually dragged a sledge like this no less than 1,723 miles in unimaginably worse conditions," says Branagh. Watching the picture critically, director Charles Sturridge smiles. "That's him," he realises with satisfaction. "Ken's got him just right—he really is the man who never gave in." Kenneth Branagh had become Ernest Shackleton.

The story begins in October 1914 when a rugged little ship called Endurance set sail on its journey from Buenos Aires. On board, 28 explorers, scientists and seamen led by Sir Ernest Shackleton were set on a daring expedition in an attempt to cross the Antarctic coast to coast via the South Pole. Nearly two years later, three ragged, reeking, scarecrow figures arrived at a Norwegian whaling station on South Georgia, a remote island near Cape Horn. One reached out his grimy hand. "My name's Shackleton," he said. He related an incredible story. Trapped in pack ice the whole winter, Endurance had been crushed and sunk. For five months, the expedition camped on the floating ice then rowed through blizzards and gales in open lifeboats to reach desolate, uninhabited Elephant Island. Leaving 22 men under two upturned boats, Shackleton set out to get help. With five men, he voyaged 800 miles in a leaky boat just 22 foot long across the world's stormiest ocean—in winter. Miraculously, they made it to South Georgia but landed on the wrong side of its high, unmapped mountains. Three men close to death were left in a cave. Shackleton and the other two, with no tents or sleeping bags, and with boat screws as spikes on their boots, took incredible risks to reach the whaling station. Then in a succession of small ships which were borrowed from the Norwegians, Chileans and the Falkland Islanders, Shackleton battled against the pack ice to return to rescue his men. On the fourth attempt he succeeded, his hair now grey from the strain. He wrote jubilantly to his wife: "Not a life lost and we have been through Hell!" Shackleton found lasting fame as the leader who kept his men together when all hope seemed lost—he never gave in. "When you are in a devil of a hole and there seems no way out, get down on your knees and pray for Shackleton," said one polar explorer.

In April 1999 it was Charles Sturridge, who made his name directing Brideshead Revisited and was now finishing his new blockbuster Longitude, that was in the "devil of a hole" and praying for Kenneth Branagh. Shackleton's masterful feat of rescue was enjoying a renaissance and Sturridge was determined to write and direct a four-hour TV epic about the adventure of the century. But who would play the formidable yet romantic figure described by his own men as "the greatest leader on God's earth bar none"? "At every key moment of Shackleton's struggle", says Sturridge, "it was the way he talked to the men that changed their mood and made them go on. The one actor with the voice to make people believe what he said and the physical presence to override their hardships and inner terrors was Branagh." Sturridge made his pitch to the actor at a Chinese restaurant in London. "I can't give you a script," he explained. "Unless you agree to do it, there's no point in my writing one." In true Shackleton spirit, Branagh made up his mind in about one second. The hardiness of gentlemen explorers had beguiled him since he was a boy. "We're going to shoot on the ice of course?" Branagh enquired. "Definitely—the real thing," Sturridge replied.

One day last April, Norwegian supply ship Polar Bird edged into the floating pack ice east of Greenland. On board with Branagh and Sturridge were 21 actors and a film unit of 88. Straight away the powerful ship got jammed for four hours. On deck in the grey Arctic light, Branagh shivered as he gazed over the seemingly limitless expanse, and not just from cold. The ice felt like a different planet, yet its mystique fired his imagination. "In a small way I sensed what lured Shackleton to the south," he says.

Shackleton had explored the coldest, driest, highest and windiest continent on this earth. While a kitchen freezer's temperature is around minus 23 degrees C and human skin freezes at zero degrees, an average winter day in Antarctica is minus 60. As Polar Bird nudged up to a huge ice floe and landed the actors and film crew on it to begin shooting, Branagh's physical resemblance to Shackleton was startling. Both five foot ten, both aged 40, they shared the same stocky frame and an air of physical toughness.

Like Shackleton, Branagh is breezy, firm-jawed, with a furious energy and full of banter. When he dyed his tousled, sandy hair dark brown and plastered it down with a centre parting, like Shackleton's, producer Selwyn Roberts had to do a double take. "Ken was Shackleton in the flesh."

There were uncanny parallels in their lives too. Both spent their early years in Ireland and share a romantic Celtic streak, Shackleton reciting Browning as he marched across the ice, Branagh with his passion for Shakespeare. Both moved to suburban England as boys, Shackleton to Dulwich, where his father was a doctor, Branagh to Reading to avoid the Troubles. And both of them found early success. Shackleton sailed in windjammers, became a master mariner, and in 1901 joined Captain Scott's first expedition to the Antarctic. Branagh caught the acting bug in Oh! What a Lovely War at school. As a teenager he hitch-hiked to Stratford-upon-Avon with a tent to see Royal Shakespeare Company productions and, after drama school, became one of its leading players.

They also shared an inner recklessness. In 1907 Ernest Shackleton launched his own expedition independently of the establishment and fought his way to within 97 miles of the Pole. Branagh, on landing one of the plum jobs in his profession as a young star of the RSC, rejected it and instead opted to do Shakespeare his own way on stage and in film.

Branagh is disarming about the extent of his resemblance to Shackleton, but when filming on the ice he too showed qualities of natural leadership. Shackleton led from the front, working harder and longer than anyone else, and insisted on taking his turn at fetching food for the men in his tent. On board ship, Branagh washed up, carried stuff and helped to cook. "On the ice it was often cold, wet and exhausting and Ken's leadership was crucial," says Sturridge. "Had he been the kind of actor to say he couldn't go on, the others would have done the same and the project could have folded."

Other actors spotted the same quality in Branagh. "When we were rowing through the ice floes in leaky boats, far from Sturridge and his camera, Ken took command," says Lorcan Cranitch, who plays Shackleton's second-in-command Frank Wild. "He kept us rowing hard and cheerful, while at the same time having an uncanny knack of positioning the boats just where the director wanted the shot."

As Endurance was crushed by the ice, Shackleton stood on deck casually smoking and was last to leave. In a giant studio, Sturridge created the scenes of buckling decks and whip-cracking beams on a 60-ton hydraulic stage that gradually tilted a mock-up of the ship, Titanic style, into a huge tank of water.

After abandoning ship, Shackleton called the frightened men around him on the ice and made a characteristic speech. "We'll all reach safety in the end if you work your utmost and trust me," he said.

Throughout the first night on the ice, while his men slept, Shackleton patrolled the floe. Suddenly it cracked, splitting the camp in two. He blew a whistle and everybody shifted hastily to the same side. Eerily, the same thing happened to Branagh's crew. Their Sunday lunch was interrupted when the huge floe, on which they had been filming, suddenly split from end to end. Twelve men in survival suits dashed on to the ice to rescue 200 pieces of gear.

For Branagh and the cast it was a mercy that they could act out the hardships of Shackleton's hell on ice rather than suffer the real thing. The original explorers had to wake in pools of icy water melted by their body heat; chomp through monotonous meals of penguin, seal and eventually even their dogs; and endure being pounded, drenched and frozen almost to the point of insensibility on the voyage to South Georgia—16 days of which Shackleton described as "supreme strife amid heaving waters".

For Branagh, the three-week shoot gave him insights into Shackleton's immense emotional strength he'd never have gained in a studio. "You only have to be cold on the ice for half a day and witness your own people scrapping over a Mars bar to realise the effort he must have made to keep his men calm and cheerful," says Branagh. "His indomitable spirit sprang from an optimism that set men's souls on fire."

Every day for five months while camped on the ice, Shackleton visited every tent to play cards or tell stories. Even when his sciatica was so severe that he had to be helped out of his sleeping bag, he never missed a visit, whatever blizzard was blowing. His real reason for going to the men was to check their emotional temperature. "He could read people well and sense the ones who were getting homesick, the ones suffering severely and not letting on, the ones likely to stir trouble," says Branagh. One night the actor was being filmed trudging through the snow, going from tent to tent, leaving every man inspired and cheerful. "The camera was a long way back, snow whizzing around, just the vastness of the ice-scape beyond the tents," says Branagh. "I was struck by the aloneness of it and what a drain it must have been to find the energy and keep going at the end of the day when the others had eaten and gone to bed."

Above all, Shackleton was a master of tiny things that have a massive impact. After abandoning the Endurance he told every man to cut his personal possessions to two pounds' weight. To set an example he threw his gold sovereigns down on the ice, then a prized cigarette case and (after tearing out three pages) even the Bible that the queen had given him. But when the meteorologist threw down the banjo which he had played during so many singsongs, Shackleton handed it back. "We're going to need this," he said.