The formula is tragedy; for an hour and forty minutes, Casey Affleck’s documentary shows his brother-in-law, Oscar-nominee Joaquin Phoenix, in the grips of an existential crisis. His highly successful acting career, he declares, is completely unfulfilling. All in all, “I’m Still Here” demonstrates a tragic fall, a leap into uncertainty’s abyss.

In the first scene, for instance, a young, swim-trunked boy stands on a rock overlooking a pond — presumably Phoenix in Panama — and jumps down. The presumed home video mimics the audience’s voyeurism; after the plummet, a man whose face we never see applauds.

In the second scene, we see another home video, of a young Phoenix, presumably, performing à la Jackson Five with his siblings, which transitions to video clips and images of print articles from just about every mass media and entertainment news source. His life is framed as a sort of a spectacle, served for our consumption.

Finally, the Phoenix of “I’m Still Here” speaks to us. Acting makes him feel like he’s in a “self-imposed prison of characterization.” His life is “completely forged,” he says, targeted toward the media and driven by its expectations. He is going to quit acting forever. Affleck’s project, Phoenix makes clear, “really represents me.”

“I’m Still Here” can read as a litany of hedonistic ridiculousness: coke, hookers, chain-smoking — at one point a despotic Phoenix, moaning like an insane homeless person, commands his assistant to make snow angels: “Do the fucking snow angel!” Nothing is under his control. Nobody even wants to make snow angels with him. It’s very tragic and very sad; he’s just a person — a highly talented person — with a deeply troubled family history, and he just wants to be a rapper. He wants more out of life. He takes a risk, pursues his dream. “Is it that your dream is unattainable … or that you chose the wrong dream?” asks Phoenix in a shitty blue sweater.

In a less generous, more voyeuristic reading, Phoenix gets what he deserves. He is thankless and out-of-control, and we can only have so much pity for a rich boy who doesn’t realize how lucky he is. Lines like, “I’m still real / I won’t kneel,” show that he’s not even a good rapper — he’s actually pretty awful. He’s a self-absorbed, irresponsible, paranoid jerk with a serious drug problem, another celebrity gone wild.

After weeks of speculation, director Casey Affleck confirmed in an interview with The New York Times that “I’m Still Here” is not so much a documentary as it is fiction, a chronicling of elaborately staged performance art. This means that the two years of Phoenix’s disappearance from the silver screen, as well as his sloppy failures of public appearances — including a now-infamous barely-interview on The Late Show at the end of which host David Letterman says, “Joaquin, I’m sorry you couldn’t be here tonight” — were all carefully, audaciously constructed. It is a joke or a hoax or a performance — depends how you look at it. Nevertheless, as Affleck says, “It’s a terrific performance, it’s the performance of his career,” and this is true, if only for its unorthodox testimony to commitment and its audacious, maddening longevity.

While Phoenix’s risk-taking is certainly exhilarating — if not commendable — “I’m Still Here” is an uncomfortable film. It feels undecided. Affleck is afraid to commit, unable to point the film toward either the initiated or the uninitiated.

“I’m Still Here,” occupying the interstice between real and fake, fiction and documentary, ultimately renders the film neither funny nor touching. Regardless of whether you’re in on the joke, very little of Affleck’s mischievous-schoolboy-making-a-film aesthetic is enjoyable. Nevertheless, “I’m Still Here” raises important questions about genre, about marketing and about the construction of persona and mythology. Its greatest success in this didactical regard is its nods to celebrity culture, to today’s multiplicity of audiences. Not even the private is ever really private. Perhaps most interesting to me is that to rebel against a celebrity persona, Phoenix created yet another persona that conformed even more to a celebrity’s, artifice on top of artifice — but at least, in some way, he has control. In such a way, Phoenix’s triumph of a performance taps into a tradition of performance art that plays with audiences, crowds, personages and perceptions, and which lays them devastatingly bare, a tactic demonstrated most successfully — and poetically — by Andy Kaufman.

For those who still doubt that the film is fiction, the signs are everywhere. The lexical field is nearly air-tight. As a result, reviews of this film released before the “tell,” though honest, seem a little embarrassing. Perhaps because of this, even regardless of the film’s aesthetic value, “I’m Still Here” could be an excellent exercise in movie watching. Technically, the shift from shaky handheld camera to smooth, sleek HD — seemingly completely unmotivated — is one sign of Affleck’s disingenuousness. The onslaught of shameless immaturity, such as the copious male full-frontal nudity — also completely unmotivated — is another. In other words, “I’m Still Here” asks if you’ve been paying close enough attention. And if a close-up of Phoenix snorting cocaine from in between a hooker’s breasts doesn’t scream fiction and a long shot of an Oscar winner being shat on by his assistant while he sleeps doesn’t either, then maybe you haven’t been paying close enough attention.