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""If it's easy, I don't do it"," Terry Gilliam remarks to camera in a spot-on summary of his filmmaking ethic. ""Without a battle I don't know how to approach it"." Well, Gilliam's fought many battles in his career, but until the abortive shoot of his The Man Who Killed Don Quixote, he'd never really lost one, at least when so much was at stake, and watching the normally chirpy helmer behind Brazil and Twelve Monkeys become worn down by the collapse of his dream project makes for painful viewing - - whether you're a fan of his fantastical output or not.

There's absolutely nothing schadenfreudic about watching Lost In La Mancha, and that's thanks mainly to directors Keith Fulton and Louis Pepe's measured, non-sensationalist approach. As the documentary begins, you expect to be greeted by an intro that explains you're about to see the story of a spectacle that went spectacularly wrong. But Fulton and Pepe consciously avoid such a scene-setter, instead encouraging a suspension of disbelief by launching straight into the Making Of tale as if they - - and we - - are unaware that anything's going to go wrong.

This is a smart move. As Lost In La Mancha kicks off, you can't help but be whisked up in the mini-hurricane that is Gilliam's enthusiasm. Plus, the storyboards, script read-throughs, snatches of dailies and shots of the cast in costume (including Johnny Depp, Vanessa Paradis and Jean Rochefort, as Quixote) are used so effectively that you'll be convinced Quixote is a masterpiece in the making. Which is why, when it does all go wrong, it has such a resounding emotional impact.

There's no denying that, like Eleanor Coppola's Hearts Of Darkness (which charted Apocalypse Now's nightmarish production), there's a certain train-wreck fascination in watching a film shoot fail, but be warned: anyone capable of empathy will find La Mancha an upsetting experience. Don't let that put you off, though. It's precisely because of this that it ranks as one of the best films about filmmaking since, well, Hearts Of Darkness.

A masterfully constructed, fly-on-the-wall revelation, which not only shows what went wrong with The Man Who Killed Don Quixote, but also proves how painfully dedicated a director Terry Gilliam is.