Art prank or self-immolation? I'm Still Here claims to be a documentary about Joaquin Phoenix, star of Walk the Line and Gladiator, as he shucks his film career for a new life as a rapper, transforming himself into a pudgy, sullen, unkempt man-child. The result is mesmerizing,a portrait of life within the dehumanizing bubble of celebrity, as Joaquin (or "Joaquin") abuses his entourage, pursues P. Diddy to produce his album, cavorts and does drugs with prostitutes, and finally has a supremely awkward appearance on Late Night with David Letterman, which triggers an emotional implosion. What are Phoenix and cowriter-director Casey Affleck after--an x-ray of their lives under the spotlight of fame? An essay on the banality of decadence? A heartfelt exploration of how hard it is to escape the labels placed on us? The movie may simply be evidence of the true project, which was planting a meme in our culture--demonstrated by the abundance of parodies and caustic commentary heaped on Phoenix after his Letterman interview. Whether it's real or fake or a variation on the Borat approach of provoking real responses through fake behavior, there's no questioning Phoenix's thorough immersion in this persona. Whether it's a performance or a perverse martyrdom, it's impressive.