I'm Still Here is either the culmination of a bold artistic project or the chronicle of a man losing his connection to reality. Ever since Oscar nominated actor Joaquin Phoenix announced his retirement from the silver screen in 2008, showbiz columns have been awash with shock, speculation and derision.

Conspiracy theorists gained credence when it was revealed that the Walk The Line star intended to embark on a new career as a hip hop artist. Before long, grainy cell phone footage of Joaquin's laughable attempts at live MC performance became a viral sensation and the cries of "hoax!" rang out louder and louder.

His brother-in-law, Casey Affleck (The Assassination Of Jesse James; Gone, Baby, Gone) was on hand to chronicle the entire escapade. Fictitious or not, I'm Still Here is a gripping depiction of a man's mental instability and the heartache he causes for those closest to him. Joaquin's long suffering assistant is central to the plot, providing a vantage point from which the audience can watch him unravel. The prominence of this confidante also allows I'm Still Here to inspire reactions beyond the pity and revulsion its main character provokes.

The film begins with a playful poke at the viewer. A hooded Phoenix stalks through pitch black woodland, decrying the 'character' he has been playing for the media throughout his acting career. Now, he says, turning to face the audience, we'll see the real man. The moment is a complete non-sequitur and occurs in a location and costume that the film never revisits. It's a ballsy challenge to anyone watching: are we peering beneath the layers of Joaquin's psyche or simply indulging him in his most accomplished character yet?

Phoenix is depicted as a socially awkward oddball from the get-go, but embarks on his fledgling rap career with a pretentious and naive optimism that is almost endearing. His obviously lame attempts at hip hop are chucklesome, as are his fumbled efforts to bring Sean ‘P Diddy' Combs on board to produce his debut record. I'm Still Here delivers on its comic moments, with debut director Affleck scattering them effectively throughout.

I'm Still Here is at its best operating at either end of the believability spectrum. When Ben Stiller tries to convince Phoenix to become a supporting player in Greenberg, Joaquin's impoliteness feels plucked straight from the textbook of Curb Your Enthusiasm or The Office.

At the other extreme, the most realistic sections are the most gripping. There's a genuine tension in the air as Phoenix plays P Diddy a selection of his home-recorded rap tracks, under the mistaken assumption that Mr. Combs is ready to sign on as producer. As it dawns on the dishevelled former star that his hip hop career is destined for failure, he begins to truly unravel.

A watershed roasting on the Late Show With David Letterman tips Phoenix towards disaster. With critical, public, and financial ruin looming, he begins to lash out at those around him. Phoenix spreads the blame onto his flunkies, becoming increasingly enraged as the world at large refuses to take him seriously. Former pop star and hanger-on, Antony Langdon, bears the brunt of Joaquin's ire. The culmination of their heated exchange is a piece of scatological revenge that prompted cries of disbelief from a few fellow critics.

After a dynamite third quarter, Affleck (and potentially his co-conspirator, Phoenix) apparently realise that the film needs an ending and speed off to Joaquin's childhood home of Panama. The story ends with a downtrodden Phoenix revisiting a former swimming hole, glimpsed in an earlier home movie. It all feels very contrived, as if Affleck and co. were grasping for some pathos after crafting such a believably insane and dislikeable persona for Phoenix.

It seems fairly unlikely that the crew just happened to bring an underwater camera to Panama, on the off chance that they might need to shoot some mostly meaningless shots of Phoenix plunging into murky river water. It's a rushed conclusion that undermines the perfectly pitched ambiguity of the rest of the picture, but, despite this hackneyed ending, the film retains its status as an intriguing portrait of a troubled psyche in the limelight, which tackles the topic of its own potential fakery head on.