Whether a hoax or not, a new documentary about Joaquin Phoenix and his transition from acclaimed, brooding actor to bearded, shambolic hip-hop wannabe has captivated viewers at the Venice film festival.

I'm Still Here was directed by Casey Affleck, a successful actor and Phoenix's brother-in-law.

The guessing game about whether the film was a genuine documentary or an ironic "mockumentary", poking fun at an intolerant and narrow-minded public and press, began long before its release.

It mirrors Internet chatter after Phoenix's now infamous TV interview with David Letterman last year, in which his confused and mumbling performance prompted suspicions that it was all an elaborate act.

"There is no hoax," Affleck said after his directorial debut was screened for the press yesterday in Venice, where it is out of competition.

"That never even entered into my consciousness until other people began to talk about the movie," he said at a briefing at which he was repeatedly asked if certain scenes, and the movie in general, were genuine.

But he conceded audiences were likely to be confused. "I'm very interested to hear those sorts of reaction, and I understand how a lot of this movie could be confusing."

Affleck said Phoenix was in Venice but he was not sure if he would turn up at the red carpet premiere later in the day.

Whether real or not, I'm Still Here offers a sometimes excruciating insight into the life of a celebrity and into the mind of Phoenix.

He is at times funny and coherent, and at others childish, aggressive and paranoid as he struggles to live with his decision in 2008, which the media greeted with breathless disbelief, to give up acting and take up hip-hop.

After his Letterman appearance, Phoenix clasped his head in frustration because it had gone so badly: "I'm just going to be a god-damned joke forever," he said, before launching into an expletive-ridden tirade and crying.

The film includes footage of him seemingly taking drugs, surfing the Net for prostitutes, hosting prostitutes, diving off a stage to attack a heckler, and vomiting.

It also features rap star Sean Combs who, after several failed attempts on Phoenix's part to set up a meeting, agrees to listen to a demo of his music.

Comb's face as he listens to three demo tracks is one of the movie's highlights. Phoenix leaves crestfallen when Combs makes it clear that he will not produce his record.

Reporters asked whether Combs was in on the joke.

Affleck replied: "The role he played in Joaquin's life was to be the bearer of bad news. He was the hammer that crushed the dream. All of that is a little bit of an act."

Comedian Ben Stiller appears when he goes to Phoenix's home to ask him to consider playing a part in his recent movie, Greenberg.