(EW.com) -- On Feb. 11, 2009, when Joaquin Phoenix made his infamous monosyllabic, grunged-out appearance on "Late Show With David Letterman," it inspired an orgy of media gossip fueled by a basic question: Was Phoenix having some sort of mental breakdown -- or was his whole shambling, beatnik-derelict routine an elaborate hoax?

"I'm Still Here," Casey Affleck's fascinating and scary documentary about his brother-in-law, is powerful evidence that the answer is neither -- and that, in fact, it's much sadder.

The movie, an artful piece of exploitation vérité, follows Phoenix's self-righteous retirement from acting, his awkward attempt to relaunch himself as a hip-hop star, and his wasted retreat from human relationships.

The Joaquin we see in "I'm Still Here" is a slurry, bloated shell of his former self, with taped sunglasses that never come off and unwashed hair that's matted into crusty dreads.

On stage in Miami, during the notorious performance where he dove into the audience to brawl with a spectator who ticked him off (apparently an easy thing to do), the hapless violence that we all saw 100 times on YouTube is less shocking than the extraordinary, dilapidated numbness of Phoenix's demeanor during his abbreviated rap song.

Standing there in his shades and overly tight suit jacket, holding his cigarette as if he couldn't be bothered to put it out for his act, Phoenix, without doing much of anything, radiates such smarmy contempt that you may think: Who is this -- the second coming of Andy Kaufman's Tony Clifton? A reincarnation of the last days of Lenny Bruce?

In "I'm Still Here," Phoenix keeps saying that he wants to be a hip-hop musician, but he communicates a borderline indifference even to his own performance. And that suggests that his real mission isn't to be a rap star at all but to act out his superiority: to the curiosity seekers in the audience, to the legion of fans and movie industry colleagues whom he's turned his back on.

After all, why couldn't he try to launch a music career without giving up his day job -- the way that, say, Jamie Foxx (who appears briefly on the same stage as Phoenix in a club in Las Vegas) has done? The answer is that it's all about ''street cred.''

By trashing what he's done before, Phoenix thinks he's not just another lowly entertainer on puppet strings but a pure artist who doesn't give a damn about celebrity. (Even though his celebrity is the only reason anyone would begin to pay attention to his rap career.)

He's become the kind of convoluted narcissist who flogs his sincerity until it becomes a twisted form of phoniness.

That bedraggled desire to declare himself above and apart from everyone else is the defining feature of what now passes for Joaquin Phoenix's ''conversation.''

As "I'm Still Here" goes on, he talks and talks but never really listens, gushing forth instead in a slovenly, rambling mash-up of self-pity and self-glorification.

The look Phoenix is going for is late-period Jim Morrison (with worse hygiene), but he sounds like a Gen-X Dennis Hopper before rehab. What the film reveals is that Phoenix, whom we see repeatedly snorting cocaine (and surrounded by empty beer bottles), is less an enigmatic head case than a toxically ego-fueled drug casualty.

Some of the movie is funny in a hungover, "Entourage"-meets-Christopher Guest sort of way.

When Ben Stiller shows up, with the script of Greenberg in hand, to offer Joaquin a role in the movie, Phoenix is so blurry and out-of-it that it's clear he has never even looked at the script and has absolutely no idea which role he's being pitched. He assumes that Stiller wants him to play Greenberg -- which only caps how annoyed Stiller is to be wasting his time with this bum.

The story isn't over, though. When Stiller shows up as a presenter on the Academy Awards, in full-bearded Joaquin costume, it's now clearer, in hindsight, that he was getting his revenge.

That Phoenix's handlers -- and Casey Affleck -- enable his behavior by never attempting an intervention amounts to an ethical lapse.

Yet the film's unflinching honesty is, in the end, its own justification. Affleck uses Phoenix's descent to forge a riveting -- and, in its way, cautionary -- case study of a celebrity self-destructively addicted to his own psychodrama.

Phoenix may say that he's left acting behind, but whether he's trolling the Internet for hookers, trying (hilariously) to convince Diddy to produce his rap album, getting huffy with an entertainment journalist at a junket for Two Lovers, or lashing out at fame while still enjoying all of its perks, the movie understands that his Last Honest Man in Showbiz routine is really a performance -- even if it's one the actor himself is only dimly aware of.

The real hoax is the one that Phoenix has perpetrated on himself. It's the illusion (to quote one of his bad hip-hop songs) that he's a ''compli-f---in'-cated'' rebel rather than just another vain burnout who needs help.

EW grade: A-

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