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There are going to be diametrically opposed perceptions of “I’m Still Here.” Some will see Casey Affleck’s fly-on the-wall film about the meltdown of his brother-in-law Joachin Phoenix, as he struggles to establish himself as a rapper after retiring from acting, as a true documentary, a painful record of his emotional collapse. Others will view it as an elaborate put-on (some would say hoax)—a tale prefabricated by Phoenix and Affleck and then played out improvisationally in real-time, using the media as well as their private cameras to record not only the actor’s descent but the world’s fascination with it. In this sense it would do “Borat” one better, fooling not just a few people at a time but everybody at once.

Critics like the redoubtable Roger Ebert have already posted reviews following the first point of view. I incline to the second. The credits, ascribing the script to Affleck and Phoenix, support that idea. So do the edge-pushing pasts of both of them (Affleck, after all, co-wrote “Gerry,” Gus Van Sandt’s wonderfully ambiguous minimalist flick.) And the final crawls also identify some players as actors—including Antony Langdon, who plays Phoenix’s aide, whom he comes to suspect of leaking stuff to the press, and Tim Affleck, who plays Joaquin’s father (though it isn’t clear whether he does so in the early home movie footage, the concluding wraparound material, or both). One also begins to wonder about the spontaneity of the infamous David Letterman appearance (shown here), or Sean Combs’ material as the guy Phoenix approaches to serve as his producer. Most suspicious are the bits by a pontificating Edward James Olmos and a bewildered Ben Stiller, which have a strangely prepared feel. (If the latter was staged, it certainly puts Stiller’s Oscar bit in a new context, making him a multiple conspirator.)

Of course, I could be wrong, and as slightly off as many sequences feel, maybe they’re genuine. Alternately, even if the project began as a scam, just keeping it up might have taken Phoenix so deeply into his role that he wound up to some extent fooling himself. Stranger things have happened.

But while it’s interesting to speculate about what’s real and what isn’t in “I’m Still Here,” the important point is that the film is utterly riveting. If Phoenix isn’t acting, watching him deteriorate is wrenching; if he is, it’s a great performance, and one can look forward to the day when he’ll shed that beard, and the extra poundage, and do another part that doesn’t carry his own name. Whatever the truth, moreover, one can also look forward to whatever Affleck might choose to do behind the camera in the future. With “Gerry” and now this, he’s shown himself as intriguing a filmmaker as he is an actor.

And whatever the mixture of fact and fiction at work in it, the picture raises provocative issues about the nature of celebrity, the character of our media culture, the underpinnings of the movie business, the world of hip-hop, and even the conventions of documentary filmmaking itself. (Surely a shot that recalls the famous closing of “The Searchers” and the long, maudlin finale that creates far too perfect a symmetry with the opening “archival” footage are no accident.) “I’m Still Here” isn’t merely a fascinating portrait of a tormented individual—“A Star is Born” rewritten for our time; it’s a commentary on a culture in which every young actor apparently wants to be a rock performer and every singer an actor (don’t call him Diddly!).

This picture is one of the best cinematic puzzles ever made—a kind of Rorschach test for every viewer. But it’s also an endlessly involving work of art, whatever its genesis and development.