I don't usually like to comment upon celebrity gossip--I view Sounds of Cinema as above that kind of TMZ trash--but I wanted to share this article on Joaquin Phoenix's recent behavior because I think the author has nailed exacly what we are seeing.
There are multiple theories as to what Phoenix's public decompensation is all about. (He announced in October that he was giving up acting for good to pursue a career as a musician and has since had one disastrous live show in which he rapped inaudibly and fell off the stage.) He could be spiraling down into alcohol or drug addiction—the actor has done a stint in rehab in the past. He could be mentally ill. Or the whole thing could be an elaborate hoax, staged with the help of his friend and brother-in-law Casey Affleck, who's planning to direct a documentary that's ostensibly about Phoenix's transition from acting to rapping but will (according to theory No. 3) turn out to be the chronicle of an Andy Kaufman-style piece of performance art.
Watching Two Lovers is the best argument for the validity of the prank theory. There's just no way an actor capable of this level of precision and attention to detail can be simultaneously spiraling down some funky mental rabbit hole. (Is there?) The Joaquin Phoenix of Two Lovers is at the top of his game, going places he's never gone before as a performer, and the passion with which he throws himself into playing the lumbering, insecure, vulnerable Leonard Kraditor suggests that it's entirely plausible that the bushy-haired mumbler on Letterman's couch is just another finely calibrated invention.
To put make a comparison, here is Andy Kaufman with Jerry Lawler on David Letterman talking about Kaufman's wrestling injury and the feud between them. We now know this to have been an entire setup, as dramatized in the film Man on the Moon staring Jim Carrey as Kaufman.
And here is Phoenix's interview with Letterman: