by Michael Krumboltz Joaquin Phoenix was just acting . That's the revelation that came from the mouth of director Casey Affleck regarding his bizarre documentary " I'm Still Here ." Affleck gave an interview with the New York Times admitting that the entire film was just a performance, including Phoenix's infamous mumble-filled appearance on "The Late Show With David Letterman" back in 2009. In the Times interview, Affleck says Letterman was unaware of the joke.