There's nothing like a little imminent jeopardy to concentrate the mind. For Kenneth Branagh, it was on the frozen wastes off Greenland as he prepared to shoot a crucial scene for this winter's two-part Channel 4 television epic, Shackleton. Months later, back in this country and sitting snugly in the warmth of C4 headquarters, where the only ice available is for drinks, Branagh graphically recalls the moment: 'The director Charles Sturridge planned for a wide shot where Kevin McNally, Mark McGann and I have to walk to the edge of the ice to determine its status and then return to our ship, The Endurance. It meant we would walk quite a long way from the camera but the ground, as it were, had been tested thoroughly. There was then a heated discussion about whether we should break for lunch. It was decided we must, which made us, the actors, rather annoyed because we wanted to get on with it there and then.

'As we were going back up the gangplank we looked back to where we would have walked and suddenly there was a massive fissure between where the camera was and where we would have been. In the next quarter of an hour, that bit of berg was well on its way. 'We probably could have dealt with it, but it was a stark though useful reminder of the sheer volatility of that environment.'

Last month Branagh won his first ever Emmy award for "'Conspiracy', playing an altogether different breed of historical figure - the nazi's nazi, Rudolf Heydrich, an architect of the Final Solution. Here Branagh is portraying everybody's hero, Sir Ernest Shackleton, in a £10 million recreation of the adventurer's amazing 1914 expedition to the South Pole. Setting off on the day the Great War broke out, Shackleton and his entire crew first survived the sinking of their ship beneath the ice before enduring a terrifying but ultimately successful two-year battle for home.

To give this Boy's Own tale extra poignancy, within six weeks of their return, the first crew member was dead in a war which they all believed would be over by the first Christmas after they set off for the Antarctic.

After last year's acclaimed Longitude - Channel 4's millennium blockbuster which won five Bafta awards including best drama serial - you'd think that writer-director Sturridge would have avoided all matters maritime. But he kept hearing mention of Shackleton, and then he met American author Caroline Alexander who'd written a book about the expedition following an exhibition of photographs by one of the survivors, Frank Hurley. 'That,' said Sturridge, 'kind of fixed it for me.'

So the 50-year-old filmmaker immersed himself in the research, knowing he had a transmission deadline just over a year away and had to deliver a four-hour film. He was just one of many directors interested in the explorer. There was, on the stocks, a lavish Hollywood studio project to be directed by Wolfgang Petersen (with Russell Crowe signed up for Shackleton); Working Title apparently had a script, and the BBC, which had previously done its own four-hour version in 1983, was also keen to reprise the story. More recently, there has been the release of a spectacular Imax version of the expedition.

'From the beginning I said to the financiers that they should expect a film to come out,' says Sturridge. 'And did they mind? Because I certainly didn't. It just meant we had to concentrate on making ours better than anyone else's. The competition was, if you like, a sign of interest in the story. It should, I added, be taken as a positive, as something definitely not to frighten us off.'

Even before he began to write, Sturridge had it clearly fixed in his own mind that Branagh - like Shackleton, Irish-born and aged 40 at the time of the expedition - was the man for the role. 'I didn't know Ken at all when I rang him up to take him to lunch. I didn't tell him properly beforehand what it was all about. When we met, at a Chinese restaurant, I told him that although I hadn't started writing yet, he was the person I wanted to play Shackleton. Now if it had been me, I would probably have said, "Very interesting, let me know how you get on with the script and, of course, I'd like to read it". With terrific grace, he said straightaway: "What a great idea, I'd like to do it."

'His first question to me was: "Are you going out on to the ice?" When I said yes, he replied, "Thank God for that." So we'd somehow managed to cover the two main points even before the spring rolls arrived. From that day, Ken never wavered in his commitment to the project,' says Sturridge.

Branagh is making his first British TV appearance for more than five years - not counting his narration on the Walking With Beasts and Dinosaurs series. 'The combination of the obvious fire in Charles's belly and his vast knowledge of the subject made me think "I'm in safe hands here",' says Branagh.

Playing the title character in what may be the network's most expensive ever drama was 'irresistible', says Branagh. He adds: 'TV's the best place for this story which takes place across two years and for a character who is clearly full of quite compelling contradictions. In photographs he looks rather serious and austere. All proper accounts suggest a tremendous larky charm.'

So with some practical tips about filming in the cold gleaned from Sir John Mills - who'd played Scott of the Antarctic more than 50 years earlier - Branagh and Sturridge and the rest of the cast and crew set off to spend five weeks on a Norwegian icebreaker.

Cold logistics precluded any prospect of a South Pole shoot. Will polar buffs be able to spot the difference? 'Visually there are two basic dif ferences,' explains the director. 'The geographical construction on the Weddell Sea in the Antarctic often results in huge amounts of pressure forcing ice into extraordinary shapes. As it happened, the ice we were working on had come from way up north and was very contorted. We were lucky to have some very fluky conditions.

'Harder for us to deal with was the wildlife situation. There were no penguins, and we weren't, as you can imagine, allowed to take any in with us. So you can say they have to come into our story artificially. On the other hand, we had to keep the polar bears out.' For Branagh, an admitted adventure junkie, just trying to get close to Shackleton's skin was thrilling enough: 'During my research I heard two tiny bits of his voice - it was higher pitched and straighter than you'd imagine. Then there was a visit to Dulwich College where he went to school, and I also got to meet his granddaughter.

'I spent an evening eating in the wardroom of The Discovery, a reminder of what his ship might have been like. I even touched some of his artefacts. When you've actually held the cooking pot that was taken across South Georgia on that Herculean trek, it gives you chills.'