Terry Gilliam is without doubt a singularly gifted visionary and an irrepressibly maverick filmmaker. However, the 66-year-old American former Monty Python animator also appears to be cursed with dreadfully bad fortune. The debacle with the financiers that plagued the production of The Adventures of Baron Munchausen, nearly bankrupting its director, might have been partly of Gilliam’s own making, but the biblical flood that washed away the Spanish set of the ill-fated and still unrealised The Man Who Killed Don Quixote was certainly a case of force majeure. As was the crisis that befell Gilliam’s latest flight of fantasy, The Imaginarium of Dr Parnassus, which he was half way through shooting when his leading man Heath Ledger died of an accidental overdose of prescription drugs last January.

‘I just said, “I don’t know how I’m going to make this thing work”, Gilliam recalls. ‘I was too distraught to actually work out what to do. But everybody around me said, “No, no, you have to carry on, you have got to do finish the film.”’

No stranger to profound misfortune, Gilliam and his co-screenwriter Charles McKeown (who previously collaborated on Brazil and Baron Munchausen) hastily completed some script rewrites, and then the director called on some actor friends to finish what Ledger started by playing different facets of his character (which in the context of the film’s series of alternative fantasy worlds seemed to make sense). Johnny Depp (the man who would have killed Don Quixote), Colin Farrell and Jude Law duly arrived to do their bit for the increasingly chaotic production and eventually the film was finished.

‘Their willingness to help rescue the film and Heath’s last performance was an incredible act of generosity and love,’ Gilliam says, ‘and, as a result, the film is even more special. This is a different film than the one we began. It’s strange, but the forced solutions may have focused us into creating a better film. All in all, it’s a bit more magical.’

Set in present-day London, the film tells the story of immortal showman Dr Parnassus (played by Christopher Plummer) and his travelling Imaginarium, a theatre that looks like a Victorian toy and which allows audience members to journey into worlds created by their own imaginations. Parnassus’ longevity, however, came at a cost – the soul of his daughter (Lily Cole), forfeit to the devil (Tom Waits), with whom the doc lost a bet, on the girl’s sixteenth birthday. Desperate to save his beloved, Parnassus renegotiates his wager with old Nick so that the girl will be now be won by whomever first seduces five other souls, and it’s here that Ledger’s charming stranger, Tony, enters the fantastic narrative.

If that plot summary, with its blend of mysticism, metaphysics and theatrics, sounds like a compendium of Gilliam films then that’s because in a way it is. Not having directed from a self-penned original script since Munchausen, Gilliam returned to his roots to dream his latest effort up from scratch, and the result is a morality play that represents a hodge-podge (not in a bad way) of the filmmaker’s thematic preoccupations. ‘The theme of imagination is central,’ says McKeown, ‘the importance of imagination to how you live – and that’s very much a Terry Gilliam theme. For some time, he’s taken other scripts and books and made them his own, but this goes further than what he’s done recently. This is more his thing. This is more a Terry Gilliam film than there has been for some time.’

‘I’m not sure whose autobiography it is,’ says Gilliam. ‘I thought it was vaguely related to mine, but I’m not sure any more. It’s about the struggle of creative people. They try to inspire others to appreciate the truth of the world, but most are not successful - that’s the reality. So we have a tragical/magical idea in the film - a group of extraordinary people in an amazing theatre, traveling round London, but nobody’s paying attention to them. I am convinced that in the modern world people don’t see what is really important anymore. There are really extraordinary and important things happening out there and nobody is paying attention.’

Getting people to pay attention is something that Gilliam has had mixed experiences with. Convincing the suits in Hollywood to green-light his wildly imaginative projects has often been tricky (so much so that he bypassed them completely for his last film, the low-budget indie flick Tideland), but when a Terry Gilliam film hits the cinemas the public always takes notice. His last-but-one, The Brothers Grimm (which also starred Ledger), might not have been a box office smash, but it, like everything from Fear and Loathing in Las Vegas back through Twelve Monkeys and on to his brilliant post-Python debut The Time Bandits, remains fondly remembered by fans. That said, Gilliam would probably go on making movies even if no one went to see them, so driven is he by his creative impulse and so dogged in his pursuit of realising his dreams.

‘I do often feel I make a film in order to find out what it is I’m making!’ Gilliam says with a laugh. ‘And I was feeling my way into this film more than I normally do.’

Having triumphed against adversity one more time with The Imaginarium of Dr Parnassus, Gilliam will test fate yet again with his next project: another shot at making The Man Who Killed Don Quixote. And one suspects this time he’ll succeed.

Out on general release Fri 16 Oct.