Phoenix says his new film, a fake documentary, was inspired partly by reality TV.
"I had started watching a lot of reality shows, and I was amazed that people believed them and that they called them reality," said Phoenix. "I thought, the only reason why is because it's billed as being real and people use their real names."
Phoenix said he and Casey Affleck, who directed the film, assumed people would know his odd behavior wasn't real.
The actor apologized to Letterman for last year's interview. He told audience members that Letterman was not aware it was an act.