Is it a mental breakdown? Is it a hoax? Or is Joaquin Phoenix sincere in his attempt to quit acting and pursue a hip-hop career?

The new film he stars in, “I’m Still Here,” dives deep into the mystery surrounding Phoenix, who provides a dark and engaging portrait of an A-list celebrity in the throes of self-destruction.

“I’m Still Here” begins tracking Phoenix in late 2008, when he shocked the world by retiring from acting. “I don’t want to play the character of Joaquin anymore,” he rants to the camera. Instead, he wants to become a rapper, and his friend Casey Affleck — brother of Ben and husband of Summer Phoenix, Joaquin’s sister — wants to shoot documentary footage along the way.

The result is like “Entourage” on crack.

As he chain-smokes cigarettes, complains incessantly, and struggles to set up a recording session with Sean “Diddy Dirty Money” Combs, Phoenix comes across as an out of control egomaniac. He flakes on business commitments. He does drugs. He humiliates the two friends who go everywhere with him.

In perhaps the most vulgar scene of a rather vulgar movie, Phoenix shops on the Internet for prostitutes, gleefully orders a pair, and then waits at his window, watching for their arrival.

Phoenix comes off like an aging-Hollywood-brat version of Commodus, the monstrous and destructive young ruler he played in “Gladiator,” wreaking havoc in modern-day America instead of ancient Rome. It’s immediately obvious that Phoenix raps terribly, and one of his go-to lines — “I’m the one that God’s chosen, bitch” — is an apt slogan for his spoiled, slovenly persona.

But is Phoenix really this much of a crazy jerk, or is it all a performance? The answer determines whether the movie is a car accident in slow motion or a comedy of the blackest order.

Either way, “I’m Still Here” is a bleak and scathing indictment of celebrity self-indulgence perfectly suited to an era of Mel Gibson meltdowns and Paris Hilton cocaine arrests.

Phoenix is remarkable, either as an incredibly narcissistic ass or an elite actor delivering a unique piece of performance art.

“I’m Still Here” flashes some nifty production touches. The opening sequence, for example, shifts from camcorder footage of Phoenix performing as a child to a stylishly edited sequence of media clips from 20 years later, heralding his arrival as a Hollywood superstar.

The film also features several cameos, including brief but fun bits from Ben Stiller and Edward James Olmos, or E-Jo, as Phoenix refers to him.

“Exit Through the Gift Shop,” the recent movie about U.K. graffiti artist Banksy, presented itself as a documentary when it probably wasn’t — and was a lot of fun regardless. Similarly, the authenticity of “I’m Still Here” is questionable, but real or not, the film offers a delightfully dark and dirty look at a celebrity on the ultimate downward trajectory.