I don't want to leave this world!” gurgles David Jason. Britain's Most Popular Actor is clinging to a log in the middle of a foaming torrent of water, which swirls through his red robes and greying beard and plasters his hair across his face. “DON'T MIND ME - I'VE GOT A BOOK TO READ,” deadpans a skeletal figure in a deckchair on a nearby rocky outcrop. Slowly, Jason's grip relaxes on the log and he disappears beneath the surface.

“Cut!” barks a voice through a megaphone. A bedraggled Jason re-emerges, and is shepherded by a squad of frogmen to the edge of Pinewood Studios' 100-square metre water tank, as the huge compressed air generators that were creating the torrent wind down. It's an overcast August afternoon near the end of the 11-week shoot for The Colour of Magic, Sky One's multimillion- pound Easter adaptation of the first two books in Terry Pratchett's supernaturally successful Discworld series.

A devoted fan of the author's sly brand of fantasy, Jason made his Pratchett debut in Hogfather, which became the most watched non-sport commission in British multichannel history last Christmas with 2.6 million viewers. Now he is fulfilling a longstanding ambition to play Rincewind, the inept, cowardly wizard who serves as the rather rubbish hero of Pratchett's early novels. In this scene he is attempting to avoid being swept over the Discworld's oceanic rim and into space, which will be represented on the vast blue screen behind him. His travails are observed by the sardonic Death, whose vocal duties have passed from the late Ian Richardson to Christopher Lee, who voiced him in the Pratchett animations Wyrd Sisters and Soul Music.

The shoot has been tough for the 67-year-old Jason. Last week he was embroiled in a precarious battle with Tim Curry's Machiavellian rival wizard Trymon atop a 300-metre tower and he will shortly be fending off dragons while hanging upside down from an inverted mountain. Also, he is serving as executive producer. No wonder he looks knackered as he is helped into a bath robe and led to his trailer where, we are told, he relaxes in a hot tub between takes.

It's left to Sean Astin, who plays Twoflower, the naive tourist who accompanies Rincewind on his adventures, to extol the virtues of the man whom, though he never asks, everyone on the set refers to as Sir David. “He's hilarious,” coos the American actor, who is up to his waist in water, clad in a sodden Hawaiian shirt and sipping tea from a giant mug. “I hadn't seen any of his shows, so I watched Only Fools and Horses. I'm five seasons in and I'm psyched,” although he hasn't seen Del Boy fall though the bar yet. “Everyone asks that.”

Having played Sam Gamgee the hobbit in the Lord of the Rings tri- logy, Astin has portrayed the creative whims of both Tolkien and Pratchett. “It's a little weird,” he says. “Some guy has a brain fart and I'm wearing big furry feet for two years. Another guy has an acid tablet and I'm in a pond in the back of Pinewood.” Of course, only one of those scribes has his tongue glued to his cheek. Astin nods: “Terry obviously loves Tolkien and fantasy but he also loves to ... take the piss out of it.”

His sentiments are echoed by Curry, who is ensconced, more comfortably, in a conference room at the other end of the Pinewood complex. “Terry's big on satire and drawing conclusions in his worlds that you can take into this one,” he observes from behind his arch-villain's goatee. “I don't think class has passed him by, or the advancements of technology. Trymon is such a wonderfully double-dealing slimeball -he'd be totally at home in Brussels. I've had a lot of extremely uncomfortable pointy shoes to wear, and lots of great hats.”

This is a small-screen movie with big-screen ambition, as the contents of the vast sound stages confirm. While most of Hogfather's locations were nocturnal and urban, The Colour of Magic spans continents. The resulting two-part film, to be shown over the Easter weekend, will weigh in at four hours.

But epic scale does not entail epic solemnity, promises Pratchett. The dungeon master himself has appeared from a side door, clad in his trademark black Fedora. This is several months before he had Alzheimer's disease diagnosed and his subsequent £500,000 donation to find a cure. He is in buoyant mood. “Hogfather was more serious; The Colour of Magic is about humour,” he says in his wry lisp. “It's a buddy movie except that one of the buddies [Rincewind] doesn't want to be a buddy. It's a road movie although roads are probably the last thing they manage to travel on most of the time.”

After the success of Hogfather - which he thought was “magnificent”, although there was “not enough money” - it wasn't hard for Pratchett to sign over the rights for two more of his books. “I tried to conceal the fact that I really wanted them to do it but really would like to be paid a lot of money,” he admits. “The nice thing is that The Colour of Magic really had no plot. It was a series of episodes and we could, like a smorgasbord, pick what we wanted. So it wasn't quite the slaughter job that I thought it would have to be.”

[]Pratchett, who has a cameo in the film's opening scene, is delighted with the casting of Jason, despite many fans expecting a younger, slimmer actor. “It was mainly the book cover illustrations that did that,” he insists. “I'm very good at not describing characters. David Jason has got three amazingly good attributes. Firstly, he is an excellent actor. Secondly, he's Sir David Jason, and that name counts for something. And thirdly he's a Discworld fan and about 15 years ago he declared that he wanted to play Rincewind. I thought, ‘Wonderful!'”

So will there be more Discworld adaptations? “I shall be 60 next birthday, so why not have some fun?” he says evenly. “I'd like to think that more will happen. In fact I'm almost positive another will happen because I've signed a contract. But I'm not at this stage going to say what it is.” Like his leading man and his millions of fans, Pratchett isn't in any hurry to leave this world.

The Colour of Magic, Easter Day and Easter Monday, Sky One, 6pm

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