WITH his wide blue eyes, bushy white beard and small frame, you could say Sir David Jason was born to play a cheeky misfit wizard.

In fact, it turns out the well-loved actor has been hankering to play Terry Pratchett character Rincewind for decades, ever since he first read the comic fantasy The Colour Of Magic 25 years ago.

"I remember reading it when it first came out and I just fell in love with the whole concept," the 68-year-old says.

"The central character was this failed wizard who'd got a spell from the famous spellbook Octavo in his head. He was just such an amusing, endearing character and the driving force behind the book.

"The whole magic and invention of this world, the Discworld, just got straight to me and ever since then I just wanted to do it."

Sir David held firm to this hope and then one day, as if by magic, he got a call from production company The Mob, who are behind a new Sky One adaptation.

"I always kept this idea in my head that one day I would play Rincewind. But I never really thought that television could make it. I thought it was only something that big budgets could afford.

"It wasn't until The Mob came along and made an offer that I couldn't refuse - I've always wanted to say that! - we finally got it on the screen. They've done a brilliant job," he adds.

The Colour Of Magic combines the first two books in Terry Pratchett's long-running Discworld series - including follow-up The Light Fantastic - and takes us on a fast-paced adventure with Rincewind and his unlikely companion Twoflower.

Twoflower, played by Lord Of The Rings star Sean Astin, arrives at the somewhat violent city of Ankh-Morpork as its first ever tourist.

At exactly the same time, Rincewind is being expelled from a world-famous wizard school for failing to learn a single spell in 40 years.

The pair are thrown together after Rincewind follows Twoflower and his aptly-named gold-filled travelling case, Luggage (which has its own feet), into a local tavern.

But they soon have to flee the city and, with Rincewind as his reluctant guide, the ever-optimistic Twoflower sets out to explore a new patch of the Discworld, complete with dragons and perilous waterfalls.

Sir David says it's this unusual 'buddy' relationship that makes The Colour Of Magic so funny.

"The central thrust of most comedy in a two-character relationship is one knowing fool and one unknowing fool," says David, citing Laurel and Hardy as good examples.

"You must have a balance, an opposite, like a ying and a yang. Where Rincewind and Twoflower succeed is that Twoflower is a totally naive tourist and Rincewind is very streetwise."

The first instalment of the two-parter, to be shown over the Easter weekend, sees Rincewind and Twoflower stealing horses to escape a burning city, riding transparent dragons and swimming for their lives to avoid being sucked over the edge of the world. All impressive stunts for an actor nearing his 70s.

"There were certain elements that we obviously didn't do, because I can't really ride a horse that well," admits Jason. "But basically everything else, both Sean and I did at least half if not three quarters.

"The bit where they go over the edge of the world, that was us. We spent two days in a tank in Pinewood and I have to tell you it was bloody freezing! I wouldn't recommend it to anybody.

"I'm quite adept in water because I spent many years diving, so when this idea came up, getting in the tank was no problem for me. But what they didn't tell you was that not only will you be in four feet of water, but you will have a rubber dinghy behind you with very powerful outboard motors creating waves.

"It looks very dramatic, but it was probably one of the worst things that I've had to do. You do it at least a half-dozen times and you've got your costume on that is full of water and weighs a ton," he explains.

And then there's the upside-down swordfight. The actor says: "In my innocence, I thought 'We'll do it standing up, have a swordfight, and they'll comb your hair back and make it stand on end and then they'll reverse the film. Then it'll look like you're upside-down', That's not what happened.

"One day we were starting to film and a stunt co-ordinator said: 'David, have you got 10 minutes? We'll try the harness on you'. And that was the first that I knew of it.

"It's not a nice experience. I thought it was bad enough nearly getting drowned, but being hung upside-down like the last chicken at Sainsbury's, I wouldn't do that again in a hurry!"

The father-of-one is a British telly institution, loved for his comic characters, as well as his more serious roles. But he's also no stranger to fantasy - or Terry Pratchett adaptations, having played Albert in Sky One's Hogfather, which was screened to a record-breaking audience over Christmas 2006.

Fans of The Hogfather will find The Colour Of Magic, with its buddy theme and a sub-plot of wizard in-fighting, a lighter-hearted - and less complex - film, says Sir David.

"If you haven't read The Colour Of Magic it's going to be much easier to follow than Hogfather. It's much more fun, it's more of a road movie."

He's also full of praise for the family-friendly nature of the show and the special effects created (like Hogfather, it was shot in HD), considering it was made on a relatively small budget - one Terry Pratchett himself described as "the size of Arnold Swarzenegger's cigar bill".

"It's totally fresh for television and for the movies it's very modern," says Sir David. "This sort of entertainment is long overdue because there's nothing gratuitous in it, it's not like Rambo - what is it 257 deaths before the interval?

"It's for everyone, Rambo lovers as well as mums, dads and our little ones are loving it - Sean's little ones and my little one, so I think this will attract a lovely family audience and that's what we made it for."

While the production team are already set on adapting another Terry Pratchett book - Going Postal - for the small screen, it seems we won't see Sir David back on Sky One for a bit.

Colour Of Magic director Vadim Jean has made a nice football analogy about the actor having to "play for another team" for a while - it's clear he's just as much in demand as ever.

The Colour Of Magic is on Sky One and Sky One HD over the Easter weekend.