Geoffrey MacNab: An unreal movie for an unreal era

So we were tricked all along. Joaquin Phoenix isn't as flaky, self-destructive and as thoroughly obnoxious as he appears under that Charles Manson beard in his brother-in-law Casey Affleck's bizarre film about him, I'm Still Here. That is a relief.

Admirers of Phoenix as an actor, who might have been appalled by his behaviour in the film, can now laugh off his antics a supreme piece of Method acting. They can sigh in relief that the film's most grotesque scene – shot in murky light – in which Phoenix's sycophantic English friend Antony defecates on his head was a set-up. Give Phoenix credit: he is utterly convincing as a man who seems to have been woken up by a shower of shit.

Where this all leaves Affleck, though, is another question. As a filmmaker, he is as shifty and two-faced as his most famous character on screen – the coward Robert Ford who kills Jesse James in Andrew Dominik's The Assassination of Jesse James. Like Brad Pitt's outlaw hero in that film, many viewers will now feel they've been shot in the back by a low-down varmint.

Affleck's film is – The New York Times tells us – a piece of performance art and a Hunter S Thompson-style gonzo stunt. Some are bound to feel the director is demonstrating contempt for the audience. In Venice last week, he refused to admit the film wasn't real. When he was asked why what appeared to be a documentary had a script credit, he wriggled out of a corner, saying: "It's a union issue. You can't have a movie that doesn't say 'written by' somebody." He seemed amused by the speculation that his film had engendered.

By speaking out now and saying that Phoenix was giving "the performance of his career", Affleck risks lessening the impact of his own movie. What made it so tantalising and provocative was its open-ended quality. At least at first, Affleck seemed willing to leave it up to the audience to make up its own mind about his movie. "I sincerely did not want to influence people's interpretations. I tell you there was no hoax," he told his Venice press conference. Now, the secret is out and the magic is quickly beginning to dissipate.

Affleck clearly doesn't want us to think that this is a feature length episode of Candid Camera. By making his revelations to The New York Times, he is clearly keen to flag up his cultural credentials. The debate about what is "real" and what is feigned isn't just confined to film. It seeps into every part of our culture. Is a cricket match "real" if a player is deliberately hurling no-balls? Is a history book "real" if the historian is inventing scenes and dialogue? How far can we trust a reality TV show when the producers use Auto-Tune to improve singers' performances? Affleck may be a trickster but at least his film is timely.