So “I’m Still Here” was actually actor Joaquin Phoenix, fresh off a speedball of celebrity and Hollywood after his turn as Johnny Cash in “Walk the Line,” doing exactly what he was good at and disappearing into a part?

My word. How were we ever to guess that?

Saying Phoenix’s performance in his brother-in-law Casey Affleck’s cutting mockumentary was, gasp, a performance is not a spoiler. It speaks to how much we expect of him, how much we buy what he’s selling, how good he is at what he does. Did anyone get angry when they realized Zoe Saldana wasn’t really a 15-foot-tall blue alien? No, because “Avatar” was a movie, duh. “I’m Still Here” is too, of course, but it elicited a reaction you only see perhaps once a decade in its first movie audiences. Was that … real? (See also: “War of the Worlds,” “The Blair Witch Project.”)

The movie follows Phoenix as his fresh, up-and-coming face turns haunted and puffy, as he grows out a ragged mop of hair and a Fugs-like beard, hides behind a gut and dark sunglasses and shuffles about in baggy pants and pretending to be a gritty rapper, his voice Jim Morrison — hoarse from freestyling over a booing crowd.

“Joaquin, I’m sorry you couldn’t be here tonight,” David Letterman says after a baffling appearance by Phoenix on the Late Show. (One can’t call it an interview, because that requires two people talking.)

One day, he’s catnip. The next day, he’s kitty litter.

One of the best things about this movie is that it’s not just a project, either, or a way for Phoenix to show off his talents. By showing, up-close, without the teasers and sound effects and celebrity guest-appearances and field trips of reality television, the woozy, reeling, acid darkness of celebrities who fall, he exposes one of the few archetypes Hollywood doesn’t like: Hollywood itself. Phoenix becomes one of the lame animals in the celebrity herd, starved for job offers and lunch dates, tumbling down a rocky hill like a rag doll after falling off the crazy train. He is Kathy Griffin on “D-listed,” or Kirstie Allie on “Fat Actress,” except no one respected them to begin with, so it didn’t hurt to laugh. In fact, we relish it.

And Phoenix has a deep, dark, solid reason for wanting to shove our faces in this dichotomy: his brother.

Remember River? He was 23 and he was going to be the next James Dean. He had spectacular hair and perfect skin and he held his own next to Keanu Reaves and was a young teen hero in “Stand By Me.” Or, at least, this was the narrative “Entertainment Tonight” decided was the truth after he died, young and tragic and high, on the sidewalk outside the black bricks of the Viper Room. As for younger brother Joaquin? No such sympathy. The world acts with scorn when Joaquin takes a swan dive off the peak of his fame, spits on him when he pisses it away in public, as though we had something invested in this man’s career, as though he owes us something. And, had he gone just a little bit further, had he overindulged in a few more bumps and burgers (tofu, of course), the commentators would have been sober, mournful, reverent.

“I’m Still Here” isn’t a prank. It’s a disgusted rebuke of a culture where the two options are “be worshipped” or “die,” where scorn comes not when you plummet to your death, but when you survive the fall. When you’re still there.

Ashley O’Dell writes about movies that aren’t in theaters anymore.