Affleck captured his friend Phoenix on camera as he quit acting and embarked on a rap career. The resulting film, I'm Still Here, contains graphic scenes of drink and drug abuse that Affleck insists is genuine.
"I can tell you that there is no hoax. That never even entered into my consciousness until other people began to talk about the movie," The Hollywood Reporter quotes Affleck as saying.
However, the director did admit that his documentary is likely to leave some audiences confused.
I'm Still Here will get a limited US release from Friday before arriving in UK cinemas on September 17.
> Click here to watch the trailer for 'I'm Still Here'