I'm Still Here (15), Casey Affleck, 108 mins

Starring Joaquin Phoeni

Watching Casey Affleck's I'm Still Here, you're faced with two possibilities (or at least you were until three days ago when Affleck came clean about the true nature of his film). One possibility was that the film was a documentary, in which case, it would be a disturbing one – not to say intrusive and exploitative. Affleck has turned his camera on his fellow actor – and brother-in-law – Joaquin Phoenix, at a critical moment. Acclaimed for his starring roles, Phoenix decides he's had enough – of acting, of fame, of shaving – and tells the world, from behind a wild thicket of beard, that he's leaving cinema to become a rapper.

Phoenix seems to be undergoing a breakdown, as becomes apparent in a notorious chat-show appearance. Shambling and monosyllabic, he becomes the butt of David Letterman's one-liners: "Joaquin, I'm sorry you couldn't be here tonight." It's a sorry story, and Affleck captures every pitiful moment, from Phoenix's earnest attempts to impress rap mogul Sean "P. Diddy" Combs to his nervous backstage vomiting. Affleck goes where few directors would have the nerve to, and where no normal brother-in-law would have the decency to.

Well, that was one reading of I'm Still Here, if you chose to buy it. The other possibility, however, was that the film was a put-up job, recording and perpetuating an elaborate performance project that Phoenix has apparently been committed to for something like two years.

There are certainly points where pretence and reality intersect. Phoenix, it's clear, genuinely is sick of the acting treadmill, and sufficiently weary of playing the matinée idol to let his physique go to seed: shirt off, he carries a Brando-esque layer of flab. But as for rapping, he's really taking the piss. His self-pitying lamentations, delivered in stumblebum monotone, are at best borderline-competent, although not so inept that they instantly reveal themselves as parody.

Allegedly peeling back the Hollywood façade, Affleck shows us the life of a pampered star as, supposedly, it really is. We see Phoenix insulting his assistants, nuzzling a hooker's breasts, laying out cocaine while barking, "Who's the nigger in charge here?" He raps in clubs, barely able to finish one number without throwing a strop. The star's attempts to meet Diddy, with a view to collaboration, make for excruciating pathos. When he finally gets an audience in the producer's studio, Combs coolly shreds Phoenix's hopes.

However, the showbiz gossip circuit quickly sniffs the possibility that Phoenix is pulling a hoax, and that very suspicion is built into the film. The star takes tearful umbrage on being accused of fakery, then turns viciously on his assistant Antony, accusing him of spreading rumours. Eventually, even this faithful yes man has enough and takes his revenge, in the film's most scabrous joke.

There are plenty of clues to what's really happening – including the credit "Written and produced by Casey Affleck and Joaquin Phoenix". Many scenes have a manifestly staged look – notably those featuring Ben Stiller, who drops by to offer Phoenix the best-buddy role in his own latest film. All in all, this mock-vérité comedy of Tinseltown folly suggests a rap-inflected answer to Larry David – a sort of Curb Your Enthusiasm, Muthafucka. Phoenix's often horrifying antics also recall the daredevil public-baiting stunts of the late comedian Andy Kaufman. This is career suicide as performance art.

Phoenix may have comprehensively dismantled his star aura, but he hasn't done himself any harm as a performer. Watched in this context, the Letterman appearance comes across as acting of the highest calibre, Phoenix's zombie impassivity never cracking for a moment. In the scenes with Combs especially, Phoenix is uncomfortably funny: his "Joaquin" character is a self-indulgent boor, but likeably vulnerable too.

It's a good performance, if one-dimensional. But in the end, you wonder what Project Phoenix actually reveals, other than that stars are infantile, fame lets you get away with murder, and the media machine is gullible. With or without the subsequent revelation, Phoenix comes across as a privileged prince. Look – he seems to be saying – I'm so compelling that I can make myself a monster with impunity.

Without doubt, the world of celebrity needs to be demystified, to break our rapt obsession. But when that world is criticised from within, there's inevitably a taint of poor-little-rich-kid pleading. The film's coda certainly plumps for lonely-at-the-top pathos. At the start of I'm Still Here, we see a video clip of a child jumping into a Panama waterfall – apparently Phoenix's primal "Rosebud" moment. At the end, the adult Joaquin returns to Panama to share a silent beer with his dad. It's a poignant scene – except that Phoenix Snr, the credits reveal, is played by Tim Affleck, the director's father.

In the end, Phoenix's cleverest trick may have been to reveal nothing whatsoever about his real self. When three days ago Casey Affleck confessed to The New York Times that his film was indeed bogus through and through, it transpired that even the Panama waterfall clip was faked. As for who was in on the deceit, David Letterman wasn't; Phoenix's Hollywood agent was. Still, who knows how Hollywood will respond? Perhaps Phoenix really has destroyed his career. Time will tell. Meanwhile, it's a pretty safe bet that we won't hear him rapping again any time soon.

Also showing...

The Kid (100 mins, 15)

Following Nick Moran's effervescent directorial debut, Telstar, this stodgy adaptation of Kevin Lewis's misery memoir is a disappointment. It's the well-meaning but cliché-ridden story of an abused suburban boy who grows up to be a bare-knuckle boxer (Rupert Friend).

The Horde (90 mins, 18)

A French police squad has to team up with the gangsters it's hunting in order to escape from a rundown tower block infested by bloodthirsty cannibals. Yes, it's yet another zombie film, but it's one of the better ones, thanks to the visceral fight scenes, and a heroine who could take on Lara Croft and Ripley at the same time.

Just Wright (101 mins, PG)

Cinderella romance starring a bubbly Queen Latifah as a physiotherapist who is always cold-shouldered in favour of her beautiful best friend (Paula Patton). Yes, it's utterly predictable – but very sweet.

F (175 mins, 18)

Dull British horror film in which hoodies bump off the teachers and janitors working late in a school one evening.

Next week

Jonathan Romney gazes deep into Enter the Void, the latest delirium from provocateur Gaspar Noé

I'm Still Here is the subject of this week's Culture Club. Have you seen the film, and, if so, what did you think? Was Joaquin's sacrifice worth it? Leave your thoughts and comments below