No one in this super-sophisticated age wants to be the worrywart who goes diving under the bed when Orson Welles's War of the Worlds broadcast comes on the radio. We don't want to be caught out like the celebrities and politicians hoodwinked by Chris Morris's Brass Eye stunts, or the gullible fall guys set up by Sacha Baron Cohen in Borat and Brüno. We like to think we're ahead of every game. We know Elvis is alive. He's gone punting on the Cam with Michael Jackson. There's no pulling the wool over our eyes.

This collective determination to keep the egg as far from our faces as possible stems from the same recognisable social horror, the same morbid fear of the faux pas, that you'd find in something by Mike Leigh or Alan Bennett. And it is present in the response to the upcoming documentary I'm Still Here: the Lost Year of Joaquin Phoenix, which details the public and private meltdown of this brooding actor, who announced last year that he was cutting short his acting career to become a rapper. (To clarify: Phoenix is 35 years old. More to the point, he can't rap.)

A now-infamous appearance on Late Night With David Letterman, in which a taciturn Phoenix sat barricaded behind shades and crazy beard, felt like a bristly protest against the chat-show format, with its insatiable appetite for compliance, charisma and painstakingly rehearsed "off-the-cuff" anecdotes. That the whole affair might have been a prank (Phoenix was enjoying himself too much to suppress the odd indulgent smile or smirk) did not lessen the pleasure, or the spectacle. What was curious was the rush among the media, and online readers, to be the first to spot the joins, to cry foul.

Then the news broke that another actor, Casey Affleck, was following Phoenix everywhere with a camera, documenting this new career. Which brings us up to date. The LA Times reported on a recent screening of I'm Still Here for potential distributors: "It's far from the Joaquin Phoenix you're used to seeing onscreen: snorting cocaine, ordering call girls, having oral sex with a publicist, treating his assistants abusively and rapping badly. And not, apparently, playing a role -- or was he?"

Affleck has denied that the film is a mockumentary. Either way, I hope it isn't overshadowed by all this squabbling about definitions. It's largely irrelevant whether it is a hoax or not. Isn't it illuminating enough that Phoenix should choose to derail his prestigious career, even temporarily, with such tomfoolery? A genuine breakdown could hardly be any more revealing (and, we can but hope, would not have become a YouTube sensation).

Subject and director make an intriguing partnership. Affleck is married to Phoenix's sister. The actors appeared together as a pair of aimless delinquents in Gus van Sant's 1995 black comedy To Die For. Both men began their careers in the shadow of famous elder brothers -- River Phoenix (who died in 1993) and Ben Affleck -- whose work they have now arguably eclipsed. Aspersions have been cast on Casey Affleck's role as the director of I'm Still Here, but he has technical know-how, having helped to edit van Sant's Finding Forrester. "It's one of the best jobs on a movie," he said in 2003. "You get to assemble the thing. That's film -- juxtaposing images. When do you cut? What does it say if you cut there, or here?"

In his most complex on-screen work -- in The Assassination of Jesse James by the Coward Robert Ford, Gone Baby Gone and The Killer Inside Me (out in the UK next month) -- Affleck is a picture of wiry control. Phoenix, on the other hand, is a splurger, an actor so open, his work can be uncomfortable viewing.

James Gray, who directed him in three films (The Yards, We Own the Night and Two Lovers) wrote: "Looking back on our first collaboration, I'm not sure we actually collaborated all that much. I seem to remember a whole lot of torment and angst and yelling and screaming. But I also remember consistently being amazed by the emotional depth of the then 24- year-old, and I loved his feral unpredictability. He seemed ready to explode at any minute."

Even as a child performer (when he went by the name of Leaf Phoenix), he always seemed uniquely troubled. In the sickly 1989 comedy Parenthood, Phoenix's turn as a hostile, scrunch-faced kid racked with self-loathing supplied a sour antidote to all the hugs and life lessons. He still serves the same function in US cinema. He's still an antidote.

Ryan Gilbey is the New Statesman's film critic. He blogs on film for Cultural Capital every Tuesday

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