I understand that there is speculation that he may have been fulfilling the role of a character for a mockumentary. I maintain an open mind and hope that this is true but I suspect something much more serious. It is impossible to fake flat affect. Notice that his facial expressions are not still (as an actor would portray in trying to recreate this experience) but rather one can see what we call flat suggesting a physiological alteration of his facial expressions due to his mental state. He was dysarthric, a specifically thick tongue that again is difficult if not impossible to mimic. And finally there was severe motor slowing which is a yet another feature of intoxication or a severe psychiatric condition such as depression.
If you wish to see a similar syndrome just refer to Stephen Adler in episode one of Sober House and you will see the same features we witness here. If this is acting, hats off. But I have grave concerns that we are seeing something far more serious. If I am correct we can only hope he gets the help he needs before it is too late.
Joaquin Phoenix on Dr. Drew, September 2010 (starts around 7:05):