First Night: I'm Still Here, Venice film festival

You wouldn't want Casey Affleck as a brother-in-law. His directorial debut is a warts-and-all account of a year in the life of the troubled movie star turned hip-hop artist Joaquin Phoenix. The emphasis is firmly on the warts.

Affleck (who is married to Phoenix's sister) catches the most intimate moments in the bizarre day-to-day existence of his subject in the months after Phoenix shocked Hollywood by announcing his retirement from film acting. Anyone baffled by the career move will find few answers in a film that many believed to be an elaborate hoax. We see Phoenix vomiting. We see a friend he has just humiliated taking his revenge by defecating on the actor's head as he sleeps.

"Hate me or like me. Just don't misunderstand me," Phoenix pleads at one point. It's a forlorn request. Nobody understands Phoenix, least of all Phoenix himself. He has gone to seed, growing a bushy beard and putting on weight.

Early on, it is easy to be repelled by I'm Still Here but viewers' sympathies are likely to turn. In his own Quixotic way, Phoenix, who becomes a figure of ridicule during his transformation, is also a heroic figure. He is desperately trying to reinvent himself in the full glare of the media. You can't blame him for his disenchantment with the celebrity-obsessed culture that has made him so rich, famous and unhappy. Affleck is a disarming filmmaker. This seems like straight fly-on-the-wall reportage from the battle zone that is Phoenix's life. However, I'm Still Here is cleverly crafted and edited and often very funny indeed. If it is a hoax, Phoenix is giving one of the greatest method performances of all time.

By the same token, you can't believe that Affleck would expose his long-time friend to such humiliation unless the film had an ulterior motive. We have a fascination with celebrity crack-ups. This film gives us more access than the most inquisitive tabloid magazines into Phoenix's world. By doing so, it makes us question and even become a little ashamed about our own voyeurism.

At his press conference yesterday, Affleck was non-committal about the real story behind his film. He described it as being less about celebrity than about "friendship, ambition, dreams and the artist in general" but refused to say whether he had staged some of the more outrageous scenes.

Like its central character, the film is infuriating and provocative by turns. Phoenix undergoes a strange transformation from svelte movie star into Unabomber lookalike, giving disastrous rap performances. In doing so, he is breaking a taboo. You're not supposed to turn your back on your own success.

The ridicule that is heaped on him arguably reveals just as much about celebrity-obsessed popular culture as it does about Phoenix himself. The film ends on a mildly redemptive but still mysterious note.

We're no clearer at the end than at the beginning just what is driving Phoenix. Nor is it obvious what Affleck's intentions are. The movie is nothing if not inscrutable – and that's why it is bound to provoke far more ferocious debate than any conventional tale of a star's fall from grace ever would.