Casey Affleck admitted a couple months ago that Joaquin Phoenix’s retirement from acting, pursuit of a hip-hop career and descent into apparent insanity---and Affleck’s filming of this transition---was an elaborate gag. The real-or-fake question is no longer in play, but the duo’s ballsy gamble still yields big results, among them Phoenix’s genuine transformation from super-serious dramatist to ace deadpan comedian. The actor commits completely to his portrayal of a bloated, drug-addled and delusional narcissist who leaves people dazed, confused and amused in his cruel wake. It’s all here, from the dreadful rap performances to the infamously incoherent Letterman appearance, and it’s as riveting as it is hilarious. “I just want to be triumphant,” Phoenix moans in the film’s early moments. That he is.