My instinct will always be to find an interesting and satisfying creative solution to his attempts to deconstruct me."There is now talk of Leth making a documentary about Von Trier's forthcoming staging of Wagner's Ring cycle at the Bayreuth Festival. "It was really touching for me to see that film," sighs Leth. "I knew I was sitting in the midst of lost souls and human misery, but I was giving my performance... it's possible I could have had a breakdown, but that would have been an adequate solution also."As the project proceeds, Von Trier grows ever more desperate, and by the time they get to the fifth obstruction itself, he is so frustrated he tells Leth: "You will do nothing at all!", and directs the film himself. He had failed to trip his mentor up.In the second episode, Von Trier tells Leth to go to "the most miserable place on earth". So he heads to Bombay's red-light district and stages a one-man dinner party.

When dogged defenders tried to obstruct him illegally, Laudrup always shrugged them off. In the film, Leth does the same to the studs-up, hard-tackling Von Trier.The "victim" in The Five Obstructions is Leth, but the film turns out to be infinitely more revealing about Von Trier. At one stage, the director states that the "greatest gift" an actor can give him is to "screw up".Leth's first assignment was to go to

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Cuba and remake The Perfect Human "with no edit more than 12 frames long". Leth took advantage of the restrictions, shooting 1,000 scenes which he edited down into seven elegant, if staccato, minutes of footage Von Trier liked it, but felt thwarted. In the early 1990s, Leth made a documentary about the Danish footballer, Michael Laudrup. I am not into collective film-making," he says tartly.) Nonetheless, when Von Trier invited him to dinner when Zentropa was launching its documentary arm and asked him to collaborate on a film, Leth quickly agreed The title, The Five Obstructions, comes from football. While he admits he enjoyed the recognition from young Danish film-makers, he wasn't interested in signing up to Vow himself.

Nor was he supportive of the "dogumentary rules" Von Trier announced last year ("I am not a brother in any brotherhood. In the mid-1990s, when Von Trier drew up his film-making manifesto, The Vow Of Chastity, and launched Dogme, he acknowledged Leth as one of the key influences on the movement.Not that Leth was remotely clubbable. "But he was also regarded as a very strange man," Leth notes.Leth's wife at the time, an actress, disliked Von Trier intensely "There was a sadistic strain to him," Leth recalls. "He was demanding impossible things from recognised Danish actors who were much older than he was. That was already there in his student films."Von Trier and Leth were not especially close, but the older man knew that the young director admired his work. In particular, Von Trier relished the way Leth had always set himself formal constraints before starting on a project. Von Trier was a student at the National Film School in Copenhagen Leth was a senior tutor.