Word 'on the street' is that he is doing some kind of social experiment and having his buddy, Casey Affleck, getting it all down on film. A kind of social documentary, if you will, conducted over the course of a year.
He stages a 'retirement' followed by a 'new career' into hip hop and makes out as if he's having a bit'o'a mental meltdown.
Maybe the documentary was/is inspired by the other social docos such as 'Super-Size Me' and the like...only this will be 'when Oscar Nominated Actors Go Wild'?
When I first saw the Letterman interview, I cringed into my couch with awkwardness at the whole thing but when I saw it again...there were hints that he is 'there' after all and very likely knows exactly what he's trying to pull off.
Wouldn't be surprised if Letterman was in on it.
Andy Kaufman anyone?