The actor appeared dishevelled and odd as he attempted to promote what he claimed would be his final film, Two Lovers.
With a bushy beard and uncombed hair, he evoked the host's wrath by refusing to remove his sunglasses for the studio interview and chewing gum as he spoke.
Letterman even poked fun at his appearance, asking his guest, "What can you tell us about your days with the Unibomber?"
But backstage, the contrite attitude he put on for the TV cameras dropped and he crumbled in his dressing room.
Phoenix's breakdown was caught on camera by his brother-in-law Casey Affleck - and now features in new fly-on-the-wall documentary I'm Still Here.
In the film, the actor/rapper turns to his publicist, Sue Petricola, and states, "Oh my God, what have I done? I'm a laughing stock. My life is ruined."