Casey Affleck, whose acting has won him an Oscar nomination for The Assassination of Jesse James by the Coward Robert Ford, makes his directing debut with a riveting experiment. It's a speculative documentary about his brother-in-law Joaquin Phoenix, who quit movies in 2008 to become the bearded, pudgy rapper JP that David Letterman teased unmercifully on TV. I'm not sure I believed a word of this film. Actors who melt down on camera are usually, well, acting. But I couldn't take my eyes off I'm Still Here. Affleck's provocative, postmodern take on JP as half-joke, half-victim is the damnedest plunge into the dark heart of our "reality" culture since Sacha Baron Cohen invented Borat. My guess is it'll knock you sideways.