Joaquin Phoenix stars in I'm Still Here, a make-believe documentary about his abandonment of acting for a musical career. (Magnolia Pictures)

I'm Still Here puts me in mind of F For Fake, one of the last films directed by the great Orson Welles. In his playfully slippery proto-mockumentary, released in 1975, Welles profiles a couple of real forgers and fakers and then, near the end of the picture, pulls a sly hoax of his own on the credulous viewer. The point is to show how easily any of us can be gulled.

Which brings us back to I'm Still Here, Casey Affleck's fraudulent doc about the career suicide of Joaquin Phoenix. When Affleck admitted last week that his pretty convincing movie was a hoax, it immediately begged the question: What was the point?

Why would we want to watch a bogus showbiz train wreck when we have real ones like Lindsay Lohan unfolding before the public eye?

Affleck didn't offer much of an explanation and neither did Phoenix when he revisited David Letterman's talk show on Wednesday. Appearing slim, clean-shaven and articulate – in stark contrast to his notorious guest spot on the same show 17 months ago – he claimed I'm Still Here was inspired by the way people lap up the amateur dramatics of "reality" TV stars. Apparently Phoenix and Affleck planned to go them one better and provide an Oscar-calibre "reality" performance.

That's all well and good, but why make it such a painful and joyless experience? The only pleasure in I'm Not Here comes from that sick thrill of seeing a celebrity go off the rails – and even that palls after a while. Why would we want to watch a bogus showbiz train wreck when we have real ones like Lindsay Lohan unfolding regularly before the public eye?

As anyone who follows the entertainment news is aware, in late 2008 Phoenix, Academy Award-nominated star of Walk the Line and one of young Hollywood's most gifted actors, declared he was chucking his movie career and remaking himself as a hip-hop artist.

The immediate reaction was to suspect a joke – especially when we learned Affleck, the actor's brother-in-law, was also going to shoot a documentary charting this bold career move. Things got even weirder when Phoenix, in biblical beard and Blues Brothers shades, appeared on Letterman, mumbling, chewing gum and essentially acting like a bear shot up with tranquilizers.

However, after the clip was endlessly viewed on YouTube and other celebrities had fun spoofing Phoenix on awards shows, we heard little about him for more than a year. It's possible his absence helped to sow doubt about a hoax and, to give Affleck and Phoenix credit, they stage some scenes here that are so ugly and embarrassing it's hard to believe they were deliberately planned.

Phoenix performs in his hip-hop persona in I'm Still Here. (Magnolia Pictures)The film begins with a phony home movie of Phoenix as a child in Panama, hesitating, then making a daring leap off a waterfall. Cut to the autumn of 2008, when Phoenix, bored with acting, is preparing to make a daring leap into a music career. Only, the actor, who did such a solid musical impersonation of the young Johnny Cash, seems to have no discernible talent as a rapper. His songs are clumsy and ludicrous and he has the onstage charisma of a potted fern.

Affleck and his camera crew follow him on an increasingly humiliating journey. Phoenix pursues an elusive Sean "Diddy" Combs in the hope the music mogul will produce his first album and is finally reduced to behaving like a boot-licking supplicant. Having reluctantly agreed to do a press junket for his 2009 film Two Lovers, Phoenix makes his Letterman appearance, only to end up weeping afterwards in shame and despair. That scene is more believable than his later flip-out during a rap performance at a Miami nightclub, when he attacks a heckler. But the latter brings uncomfortable recollections of comedian Michael Richards's similar meltdown a few years ago, minus the racial slurs.

Phoenix grows more loopy and unlikable as the film unspools. He takes on the ratty look of a homeless schizophrenic and takes out his anger on his (understandably) small entourage. His favourite whipping boy is a weedy British musician nicknamed Ant (Antony Langdon, guitarist for the band Spacehog). A recovering addict, he's taunted by Phoenix for his sobriety and accused of undermining his boss by secretly talking to the press.

I'm Still Here is not a total bummer. There are kernels of promise in it. It might have served as a satire of egotistical movie stars, who think they can cross over to musical stardom without paying their dues. (A scene with Combs, who finally agrees to listen to Phoenix's lame demos, is excruciatingly funny.) Or, more seriously, the film could have been about the difficulties facing a successful artist who wants to set his success aside and take that leap into the unknown.

The trouble is that Phoenix has created such a whiney, self-absorbed and unsympathetic persona. And Affleck, making his directing debut, seems mainly preoccupied with raising the bar for outrageousness set by Sacha Baron Cohen in Borat and Brüno. Phoenix is seen smoking weed, snorting coke and cavorting with prostitutes. There's plenty of full-frontal male nudity, a torrential vomiting scene that will have you averting your eyes and, as a disgusting coup de grâce, a sneaky act of revenge by the long-suffering Antony involving defecation.

But Cohen had a satiric point to make in his movies, as did Andy Kaufman with his avant-garde comedy, or Banksy in his delightfully dodgy Exit Through the Gift Shop – to name a couple of hoaxers more intriguing than Phoenix and Affleck. The two have done little in this oddball film project but push the privacy invasion of faux cinéma-vérité to new lows.

They can congratulate themselves on being clever and authentic enough to deceive a number of critics – including Roger Ebert, who said he'd be "seriously pissed" if the movie was a ruse. But I suspect most people saw Phoenix's rap career as a stunt from the start and dismissed it long ago. That's why the title, I'm Still Here, sounds a little like a cry of desperation.

I'm Still Here opens in Montreal, Toronto and Vancouver on Sept. 24 and in other Canadian cities on Oct. 1.