Joaquin Phoenix has a total meltdown
During a 2008 interview with Joaquin Phoenix in which the actor was meant to promote Two Lovers, Letterman was beyond confused when Phoenix claimed that the movie would be his last, as he was moving on to become... a rapper? He was unable to answer most of the questions Letterman threw at him and seemed generally unwell throughout the segment.
Later, it was revealed that Phoenix was simply remaining in character for the mockumentary I'm Still Here, directed by Casey Affleck. However, he failed to disclose this information to Letterman.