The question hanging over “I’m Still Here” — a film documenting the year in Joaquin Phoenix’s life when he quit acting in order to pursue a hip-hop career — is whether the whole thing is an elaborate hoax, the sort of media stunt prankster Andy Kaufman might have pulled off. There is one scene, in which Phoenix’s personal assistant takes ghastly revenge on him after being berated one time too many, that made me wonder if the event had been staged.

But the rest of “I’m Still Here,” which was shot and directed by Casey Affleck (Phoenix’s brother-in-law), feels utterly real and transparent. If the movie is a fake, the filmmakers deserve Oscars for creativity. Phoenix has absolutely nothing to gain by presenting himself in this manner: As an abuser of cocaine and marijuana who parties with hookers he hires online, who pouts and frets when no one takes his rapping aspirations seriously; as someone prone to temper tantrums and bouts of self-pity who occasionally is so drunk or high that his dialogue has to be subtitled, someone who seems to have an aversion to showers and haircuts.

Other famous people we see in the movie have the same puzzled reaction to the actor’s antics. Mos Def smiles nervously and mutters an unconvincing “I think it’s good” when Phoenix tells him about his newfound rapper persona. Ben Stiller appears visibly frustrated when he visits Phoenix’s home to offer him a supporting role in “Greenberg” and is accused of playing up to the cameras. Sean Combs is initially extremely wary of Phoenix, who scores a meeting with the elusive record producer after learning that Dr. Dre and Rick Rubin were booked solid.

A scene in which Phoenix, visibly nervous and high, goes to Combs’ studio in Los Angeles and plays some demo tracks for him generates tremendous suspense: We want to know Combs’ professional opinion of Phoenix’s rap songs, because the snippets of his music we’ve heard in the movie are so awful no one could possibly take him seriously.

There are moments of humor in the film. Phoenix complains about having chosen the wrong road when he made “Reservation Road,” which flopped, while Leonardo DiCaprio’s “Revolutionary Road” snagged a bunch of Oscar nominations. He refuses to do a sound check before a performance in Las Vegas because he thinks the stagehands just want to hear his show for free. And before his first meeting with Combs, he frets about exactly what to call him. (“Mister Diddy?” “P?” “Puffy?”)

“I’m Still Here” includes the now-infamous David Letterman interview, which led Phoenix’s shaggy-beard-and-sunglasses look to become an object of ridicule in popular culture. But the movie also shows what happened immediately afterward, as the actor realizes the disastrous implications the TV appearance will have on his career; his emotional meltdown is disturbing to watch (“I’m going to be a joke forever!”).

By the time it gets to Phoenix’s performance at a Miami Beach nightclub, where he sings one song, gets into a fistfight and ends up puking into a toilet bowl, the film has generated a palpable aura of discomfiting darkness. Wealth and fame have never seemed less appealing.

Regardless of its veracity, this portrait of a drug-addled star who just wants to express himself artistically contains implications that exceed the filmmakers’ intentions. Joaquin Phoenix may not be all there in “I’m Still Here,” but he — and the movie — leave an indelible, unsettling impression.