"Honestly, it's like you slipped and hit your head in the tub," Letterman joked about Phoenix's bizarre February 2009 appearance. The 35-year-old actor appeared with shaggy hair, a beard and dark sunglasses, generally answering with confusion and one-word answers.
One of Letterman's writers, Bill Scheft, revealed in an interview with Nuvo magazine that Letterman knew Phoenix was faking it at the time. He said Casey Affleck filmed the entire episode for his documentary, I'm Still Here, about Phoenix's retirement from acting.
Check out the clip below:
Watch Phoenix's full Late Show interview Wednesday at 11:35/10:35c on CBS.