When you're a hip-hop star, life is supposed to revolve around "keeping it real". But Joaquin Phoenix's career as a professional rap artist has so far been defined by a struggle to keep things vaguely plausible.

The actor, who just three months ago was one of the most bankable stars in Hollywood, was this week forced to deny reports that his long-running effort to rebrand himself as the next Eminem is actually an elaborate hoax.

It was the latest in a mysterious series of events that began in October, when Phoenix decided to renounce acting, grow an enormous beard, and publicly adopt the persona of an aspiring rap mogul. Since then, he's been spotted bustin' rhymes in a Los Angeles dive bar, hanging out with P Diddy – with whom he allegedly has a record deal – and falling over after performing a chaotic hip-hop "set" at a nightclub in Las Vegas.

There is, however, one problem: despite having aspirations to become a major rap star, Phoenix doesn't appear to possess any of the requisite songwriting talent or vocal dexterity. Both of his performances so far have seen him booed off stage.

So badly did they go down, in fact, that many viewers wondered if they were entirely genuine.

But this week, Phoenix's PR representative, Susan Patricola, issued a statement to MTV denying that her client was conducting an elaborate spoof. She was responding to allegations in Entertainment Weekly that his recent foray into rap music might have been part of a bizarre, Borat-style prank cooked up with brother-in-law Casey Affleck.

But the suggestion that Phoenix is involved in an elaborate prank is one of the least damaging interpretations of his recent activities.

The previously stylish 34-year old has ballooned in weight, say fans, and begun dressing like a native of Skid Row. He is rarely seen out without sunglasses, and has cultivated his beard to the extent that one supermarket magazine recently compared him to "Jim Morrison's ghost". Most worryingly, it has been almost a year now since fans have seen him conducting what one might call a normal conversation.

It's a particularly worrying fact given his previous personal struggles – he endured a spell in rehab for alcohol addiction in 2005. Tom O'Neill, who writes the Envelope website for the Los Angeles Times, noted last week that he had appeared "quite out of it" on the red carpet recently. "It's entirely possible that this latest 'hoax' line is being floated by Phoenix's close pals, trying to give a rational explanation for the latest bizarre antics of a wacky celebrity," O'Neill said.

The manner in which Phoenix announced his retirement from acting also raised eyebrows.

It happened in October, when he appeared at a theatre to perform in a charity fundraiser looking dishevelled, slurring his words and fidgeting nervously. Shortly afterwards, at the premiere of his last film, Two Lovers, he explained he would instead pursue a hip-hop career.

"It's like greener pastures, you know what I mean?" he said. But an impromptu rap performance which he gave at a bar in Culver City in early November soon dashed speculation about hidden musical genius. The actor hunched over the microphone, mumbling unconvincingly, before being booed off.

He had barely improved by 16 January, during what was billed as his first "proper" performance, at a nightspot in Vegas. The three tracks he performed, which swiftly became a YouTube hit, confirmed that many in the audience agreed that Phoenix did indeed "suck".

It hardly seemed to bode well for his upcoming debut album, which P Diddy is said to be producing. No record deal has yet been signed, said sources close to Diddy, adding: "but if anyone can make Joaquin sound good, Diddy can".