A young actor with a formidable talent, he had proved his acting mettle in his Academy award-nominated role in Brokeback Mountain. Shortly after, he'd commenced filming on The Dark Knight, taking on the psychopathic part of The Joker, a role that would win him a posthumous Oscar for Best Supporting Actor. Heath was clearly an actor on the cusp of even greater things.

When Heath's lifeless body was discovered in his New York apartment, he was halfway through filming the enticingly entitled fantasy adventure, The Imaginarium Of Doctor Parnassus.

"We'd just finished shooting in London, that was a Saturday night," recalls the film's director, Terry Gilliam, 68. "I went on to Vancouver to prepare for the following week's work, Heath went on to New York and by Tuesday he was dead. None of us could believe it was possible. He was doing his own stunts on Saturday night and he was fantastic and then suddenly he was gone. It's not like seeing someone decay."

Recalling those shell-shocked days in the aftermath of the devastating news, Gilliam says, "We were halfway through and the star was dead. I was ready to give up. Fortunately my daughter [who was a producer on the film] kept kicking me as I lay there on the ground until I got up and came up with a solution."

The solution didn't prove as difficult to find as Gilliam first feared. The Imaginarium Of Doctor Parnassus is "a tragical, magical idea, which sees a group of extraordinary people travelling around London in a horse drawn theatre, but nobody's paying attention to them," says former Monty Python star Gilliam.

The eclectic troupe is led by Dr. Parnassus, played by the renowned theatre actor Christopher Plummer, who has the extraordinary gift of inspiring the imaginations of others - allowing them to physically enter their own wildest thoughts.

He is helped by his daugther Valentina, played by the model Lily Cole, making her acting debut in the film; his cynical sidekick, Percy, played by Verne Troyer of Mini-Me fame; and the love-lorn Anton, played by relative newcomer Andrew Garfield.

A tale of morality, it turns out Parnassus's magic comes at a price. For centuries he's been gambling with the devil, Mr. Nick, played by Tom Waits, who is coming to collect his prize, Valentina, on her 16th birthday. Oblivious to her rapidly approaching fate, Valentina falls for Heath Ledger's character Tony, a charming outsider with motives of his own.

In a bid to save his daugther, Parnassus makes one final bet with Mr. Nick, which sends the entire theatre troupe on a ride of twists and turns, in and out of London's famous landmarks and the Imaginarium's spectacular landscape.

It was the magical mirror, a doorway to the doctor's fantastic universe of limitless imagination, that proved Gilliam's saving grace.

"We established the principle that people can change on the other side of the mirror", says Gilliam. "Things are enhanced, are more extraordinary, more wondrous.

"We had already covered most of the scenes with Heath that happen on the London side of the mirror, but the big question was 'Do we get one person to take over the part or not?' I already felt it couldn't be just one, it was too much of a weight, so we should get several people to do it if we could," says Gilliam. "Then I just started calling my friends and a lot of people who were very close to Heath."

Johnny Depp, Colin Farrell and Jude Law stepped into the breach. They each flew out to Vancouver to play various aspects of Tony, in front of a blue-screen that would allow Gilliam and his team to create the animated epic grandeur of the Imaginarium.

The film's journey had begun back in 2006, when Gilliam had decided to write something original again with his writing partner, Charles McKeown.

"It was nice to see whether we could still do it ourselves from scratch," he explains, gesticulating wildly as he describes the process he and Charles went through.

"I first started saying, 'Oh let's do a compendium of all the things I've done before and am good at doing and forget all the other things that I'm not so good at doing'," he says smiling. "Literally all we had was this wagon, a weird travelling theatre coming into modern London and nobody even noticing its existence almost, just passing us by, and I thought that's a good start and then little by little Charles and I started building things, characters and events."

As a tale depicting the struggle of creative people who try to inspire others, its themes were understandably close to Gilliam's heart.

"Imagination is the central theme," he says. "I just like doing something you've never seen before. It's like when I was a kid looking through storybooks with great illustrations. Every page you turned, something even more extraordinary was happening. I wanted to do that on film and I think it's the kind of film that you can go back to again and again."

It's because of Heath, Gilliam says, that he allowed his actors greater freedom and the opportunity to improvise more than on any film he has done before.

"Heath was just so full of ideas and fresh dialogue and so unbelievably fast and inventive," says Gilliam. "He was still, in some sense, speeding from playing The Joker, which had liberated him in a way that he had never experienced before. He was always telling me 'I am doing things in scenes that I didn't know was inside me. I cannot believe it'."

It was while Heath was working on The Dark Knight in England that he and Gilliam discussed Heath taking on the role of Tony. "He had brought over a mutual friend, who had done the storyboards for another of my films," recalls Gilliam. "They were doing an animated musical video and they needed a place to work, so I offered them space at my company.

"One day I was in there to show my storyboards to the people who were doing some pre-visualisation work on this film and Heath was sitting there. I started the show and began explaining the sequences and, while this was going on, Heath slipped me a little note, which said 'Can I play Tony?' 'Are you serious?' I asked and he said, 'Yes, because I want to see this movie'. It was as simple as that."

Gilliam says Heath has "seemed to be with us the whole way".

"His energy, his brilliance, his ideas..." says Gilliam, trailing off. "The constant pressure on all of us was to end up with a film that was worthy of Heath's last performance. I was just nervous all the time but I think the end result is something I'm proud of and I think Heath would be too."

Extra time - Terry Gilliam

He was born on 22 November 1940 in Minnesota, USA.

He was the only American member of the Monty Python team, who have just celebrated their 40th anniversary.

Rumour has it that he got so stressed while shooting Brazil in 1995 that he temporarily lost the use of his legs.

He has turned down the opportunity to direct many films including Braveheart, Troy, Forest Gump and Who Framed Roger Rabbit.

He wrote Monty Python's The Life of Brian, deemed to be one of the funniest comedies in cinematic history.