Looking like a demented combination of Jeff Lebowski, the Unabomber and Hassidic reggae-rapper Matisyahu, Joaquin Phoenix has lately been making me ashamed that we share the same initials. The abrupt announcement of his plans to retire from making movies "to concentrate on his music career" came during the PR campaign for his latest movie, Two Lovers, which critics unanimously agree contains some of the finest acting of Phoenix's career. If I were director James Grey and my movie had been upstaged like this, I'd be furious.

Unless you've been on Mars or in rehab for the last six weeks, you're probably vaguely aware of how this bizarre story started. The 34-year-old Phoenix went straight from a morning press junket for Two Lovers - where interviewers claimed he seemed accessible, normal and friendly - to the set of The Late Show with David Letterman. There, sporting an unkempt beard and calamitous bed-hair, he gave an interview comprising mainly monosyllables, non-answers, grunts, exasperated exhalations and petulant responses to Letterman's jokes.

The 10-minute exchange was both painful to endure and excruciatingly funny - and immediately set tongues wagging. Days later, Ben Stiller turned up to present the best cinematography Oscar dressed in the same crumpled sub-Reservoir Dogs suit and hobo beard, mumbling and wandering around the stage, Joaquin-style, as co-presenter Natalie Portman chided him for looking like he'd just escaped from a "Hassidic meth lab".

Now you might have expected the Oscar-nominated star of Walk the Line - Phoenix played all Johnny Cash's songs himself, released an album of the same material, and later reprised Cash's 1968 Folsom Prison Live concert in its original locale - to have some parallel rock'n'roll career in mind, having taught himself to play the guitar from scratch for the movie. Indeed, in early 2008 he spent time in recording studios with Tim Burgess of the Charlatans and Creation Records' Alan McGee behind the desk (Burgess says Phoenix couldn't stop fiddling with his music and doubts it will see the light of day).

But this is much, much worse. Forget the Bacon Brothers, Dogstar or 30 Odd Foot of Grunts - the naff musical side-projects of Kevin Bacon, Keanu Reeves and Russell Crowe respectively - Phoenix swears he is intent on becoming a rap star.

A couple of weeks ago, shortly after the Letterman debacle/triumph, he showed up at a club in Las Vegas (four hours late, naturally), performed three rhymes of incomparably nerdish and lead-footed white-boyness, and promptly fell off the stage. Last week he appeared at another club in Miami Beach and regaled the audience with more of his rotten hip-hop (fans of Dee Dee Ramone's atrocious hip-hop solo album Standing in the Spotlight will recognise in Phoenix a kindred no-talent). He then leaped into the audience and started fighting with a fan who'd been yelling abuse at him, until security pulled the actor out of the crowd. "Bitch," he cried at the fan, "This is a fuckin' $3,000 suit! I have $1m in the bank. What have you got, bitch?"

One's first reaction was to assume Phoenix had fallen off some kind of wagon. He did time in rehab for alcoholism back in 2005, so there's a precedent. But it also brought to mind the death of his beloved elder brother, River, aged 23, of a drug overdose outside Johnny Depp's Viper Room nightclub in 1993. Joaquin was inside the club at the time, and his desperate call to emergency services was widely broadcast on American TV scandal shows. Could he now be trapped in a similar kind of narco-fuelled personal apocalypse?

Various TV doctors piped up, opining that his slurred speech and facial tics on Letterman were redolent either of intoxication or severe depression. Other people more sensibly noted that Phoenix is renowned for his consummate, total-immersion method-acting skills, and that the whole thing might be some kind of performance-art put-on.

Precedents include the late Andy Kaufman, who made a career of acting like a maniac (some would say like an asshole) in public and on TV, Spinal Tap, and Ashton Kutcher's celebrity prank-show Punk'd. And the presence of Phoenix's brother-in-law Casey Affleck (he's married to Summer Phoenix, Joaquin's little sister) at every one of these events, bearing a professional-spec digicam and missing none of his best friend's antics, lent weight to the suspicion that the whole thing might indeed be some sub-reality show stunt, sooner or later to turn up in edited form on the Independent Film Channel.

So is Phoenix up to something so visionary and/or demented that we lesser mortals cannot yet comprehend it? Or does he seriously think his Vanilla Ice-meets-Grizzly Adams schtick will net him serious ink in The Source or MixMag? It's hard to think so, unless we embrace more wholeheartedly the possibility of drug abuse or mental illness.

In which case, what is Affleck doing recording every moment of this catastrophic and very public breakdown? Affleck claims he is simply making a documentary about an actor becoming a musician - in which case, if Phoenix really is deluded and losing his grip on reality, his friend is exploiting him remorselessly. Alternatively, Affleck may believe he is making some hip-hop version of a Borat movie, with Joaquin firmly in-character at all times when the camera is running. Except that so far it's not funny, clever or uncomfortable; just mildly risible and faintly depressing.

There are no Amy Winehouse flameouts or Pete Doherty narco-tastrophes (none in public, at least), no off-the-edge-of-the-map craziness, no sense that a death watch is under way. There's also no wholesale embrace (satirical or otherwise) of hip-hop culture's tackier leitmotifs - no bespoke gob-grill, no low-rider, no exchanges of gunfire in studios, no pimp hat, no superfly bitches in tow, nothing.

Above all, there is not one scintilla of compensatory comedy to the whole business. Phoenix appears to have no sense of humour whatsoever, and so far neither does Affleck (except for that creepy smile he always wears). Which leads us to one conclusion only - that they are serious. And that would be a disaster, for it means Phoenix is throwing away one spectacular talent in exchange for career immolation and worldwide ridicule.

I take some heart, however, from the Letterman appearance. "Well, never say never," replied Joaquin to Letterman's incredulity at his retirement announcement. And when Letterman quipped, "I'm so sorry you couldn't be here tonight, Joaquin!" even Phoenix couldn't help but laugh along with the crowd, though he soon stifled it.

Call me optimistic, but I still expect to see Joaquin Phoenix's next movie in cinemas sometime in late-2010. He's too interesting to go all Britney on us yet.