I once did an interview with German director Werner Herzog in which he supplied me with no end of juicy quotes about his regular star Klaus Kinski - This man is a pestilence, and so forth. I thought Id struck it lucky until the following week, when I read a dozen other interviews in which the same bilious lamentations spilled out word for word.

Herzogs loathing for Kinski was pretty well rehearsed, and the feeling was mutual. Kinskis barking-mad autobiography occasionally takes a break from fevered sexual reminiscences to fling pungent venom at the director. It now turns out that Herzog was complicit in this. According to a new film he has made about his relationship with the actor, he actually helped Kinski come up with juicy insults for the book.

Playing in Cannes out of competition, Mein Liebster Feind - translated creakily as My Best Fiend - is Herzogs record of an extraordinary folie á deux. The duos tormented collaboration stretched over five films representing a kind of extended duel to the death - Kinskis, as it happened. The actor died in 1991, possibly taxed, Herzog speculates, by the rigours of their shoot for the African-set Cobra Verde. Herzog begins by revisiting the Munich apartment where, in the 50s, his family had Kinski as a neighbour.

Kinski would fill his room to knee-height with dead leaves, break down doors if his shirts hadnt been properly ironed, and on one occasion, locked himself in the bathroom for 48 hours while he reduced the porcelain to powder. Not just debris, mind you - powder.

All this was simply rehearsal for Kinskis antics when working with Herzog - most spectacularly, on their South American jaunts Aguirre, Wrath of God and Fitzcarraldo, the latter notoriously requiring a steamship to be pulled up a hill in the Peruvian jungle. In documentary footage of one of these shoots, Kinski throws a spectacular hissy-fit while a mystified Indian tribe looks on; according to the director, they were alarmed not so much by Kinskis fulminations as by Herzogs silence. They later offered to murder Kinski for him. An actor from Aguirre displays a scar in his head, the trophy of Herzogs tantrum with a sword.

The films subtext is that while Kinski may have been a force of nature, he met his match in the directors own, more muted dementia - after all, which one dreamed up that steamship stunt? My Best Fiend reveals the depth of Herzogs curious obsession with his star. You dont work repeatedly with someone that unmanageable unless their character responds to some need in you, and the relationship has all the hallmarks of sado-masochistic amour fou. It also displays the deep resources of denial that often accompany co-dependency. He said I was crazy, Herzog protests - then confesses that he once tried to burn down the actors house. The scariest moment comes when Herzog produces his on-set diary, and its written in the suspiciously tidy near-microscopic hand you associate with the intimate journals of people who pickle and label human livers for a hobby.

My Best Fiend harks back nostalgically to whats often considered Herzogs golden age - although his swashbuckling jaunts with Kinski rather overshadow the more introspective achievements that marked him as a leading light of New German Cinema in the 60s and 70s, among them Fata Morgana, The Enigma Of Kaspar Hauser and the chilling, under-rated Stroszek. Since the barely-seen Scream of Stone in 1991, Herzogs fiction career has lapsed - but he has been active as a documentarist, making remarkable films like the African tribal portrait Wodaabe and the infernal vision of oil wells after the Gulf war, Lessons Of Darkness.

In Cannes, Herzog has announced a return to fiction, as well as issuing a personal statement on his philosophy of documentary. His so-called Minnesota Declaration is a 12-point manifesto-cum-prose-poem that outdoes even the Dogma 95 statement for gauntlet-throwing intensity. It attacks the tradition of cinema verite as devoid of verite, comparing its practitioners to "tourists who take pictures amid ancient ruins of facts". Herzog declares: "Tourism is sin, and travel on foot virtue" (possibly an allusion to the time he insisted on walking across Germany in homage to the dying film historian Lotte Eisner). Then it gets more enigmatic: ìWe ought to be grateful that the Universe out there knows no smile; "The moon is dull. Mother Nature doesnt call, doesnt speak to you, although a glacier eventually farts."

Perhaps we shouldnt expect the document to be too coherent. Herzog claims he wrote it in 15 minutes, after going for 72 hours without sleep, then watching a TV fly-on-the-wall documentary and a porn movie and concluding that the latter came closer to truth. But the declaration belies the rumour that Herzogs passion waned years ago. He may have suffered fiction burn-out after Kinskis death, but his documentaries show a film-maker with his visionary drive unimpaired. Herzog has just re-focused his capacity for love and fury. Instead of Kinski as his beloved arch-enemy, he has simply turned his obsessive passions back on to the world itself.