My very personal take after reading other responses here - If Dave was in on the joke, fine. It’s his show. If he wasn’t, it was incredibly rude and stupid. Either way, I don’t like this kind of humor.
Suppose it is a put-on. Then the question is, what the hell for?! If he wants to switch from acting to rapping, he can do so without this nonsense. I don’t see how making oneself look like a complete asshole is supposed to help any kind of career.
To help sell this: I'm Still Here (2010) - IMDb
There are exactly two kinds of reasoning for his behavior as of late: drugs or a hoax. I’d put money on the latter. And since many sites are reporting that it’s a hoax to help his brother make a documentary, it really does seem the most logical. Especially after seeing his first appearance on Letterman as a normal, coherent guest (with stories!).
On the radio this morning our local morning show had on our local movie critic and he mentioned that Joaquin has a strange documentary coming out, he said it was strange, like Borat strange and that this is most likely going to be his alterego until then.
Thank you for that link. Especially since one of the accompanying videos next to it is of Salma Hayek breastfeeding. 
Yeh, okay, I’m sold that it’s a hoax/alter ego thing until this ‘documentary’ comes out.
Not sure if Dave was in on it. If he was, he had me convinced he wasn’t. Also, if he was, then the whole thing was improvised — which would explain the awkwardness, pauses and shifts in approach.
Not sure if it’s funny or not. I’ll reserve judgement until his movie comes out. Being weird just to be weird isn’t all that clever or hard to do, it’s just baffling and creepy. I hope he knows what he’s doing.
That kid doesn’t know how lucky he is.
That doesn’t prove its a hoax though. That just proves what Phoenix and Casey Affleck have been saying all along, that they plan to make a documentary to go along with Phoenix’s transition to being a “musician” (before it morphed into “rapper” a few weeks ago).
Secondly, the proof people use that its a hoax is that someone overheard Phoenix tell someone it was a joke. And after the handshake and the sunglasses removal at the end of the Letterman interview, I guess that’s possible too. He definitely “broke character” as it were or it could have been reflex from years of interviewing.
I’m leaning towards hoax, but I wouldn’t be surprised if Phoenix’s cheese done slid off his cracker.
Casey Affleck is Ben Affleck’s brother. The last name is a clue. He married Joaquin’s sister.
If this is a hoax, it strikes me as a stupid one. That performance was painful to watch for two minutes, I can’t imagine sitting through a two-hour version. Phoenix is one of my favorite actors of that age range and if he’s having real problems I hope he makes it through OK. But if this is a put-on, I’ll respect him a lot less. It’s not particularly clever or difficult to sit there like a clueless cipher and make others uncomfortable. (I was never a fan of Andy Kaufman’s “see how big an asshole I can be” style of performance art either.)
Yep. The whole “Is he serious, or is he putting us on” where he’s the only one in on the joke is lame.
If he was serious, well then he’s an idiot. If he was putting us one, well then he’s still an idiot since no one thought it was funny (or will think it’s funny) except him.
I just watched it on YouTube and, well, I thought it was funny! I had no idea there were hoax theories; someone just sent me a link with a “cheese done slid off the cracker” comment.
I thought it was hysterical how he wouldn’t answer. Obviously the man is able to answer questions coherently; I’ve seen him on talk shows before. I spent the whole time trying to decide if he was high or putting us on. I vote for #2. He’s an actor! He was acting.
Plus, I have lusted over him in a highly inappropriate way for years (I think I’m old enough to be his mother) … so I’ll wait for complete evidence before I confirm he’s gone around the bend, off the cracker, or whatever. 
Homeless crazy person as performance art. Not doing it for me, either. Yeah, as everyone has said, if this was Joaquin as Joaquin, he’s got problems (drugs or crazyness). If it’s an act, it’s stupid.
The way he sliced up that wacked-out weirdo Joaquin Phoenix was just pure gold. You tube has it…
Yeah, I don’t know whether Phoenix was faking it or not but Letterman was brilliant. I’ve watched this clip three times already and it just gets funnier and funnier.
Just saw it. I don’t think he was “on” anything. Total act.
Brilliant move on his part. We wouldn’t be talking about him if he gave a “normal” interview.
Madeline Kahn did this to Johnny Carson decades ago. It was fascinating but awkward then. Since it’s been done before, it was tedious to see it again. It’s like having to listen to a long joke that you’ve already heard.
I wouldn’t be here talking about it now if it weren’t for my husband who got me to watch it and told me about the thread.
I think it was an act and I found it funny, not uncomfortable at all (which is weird because I’m normally sensitive to uncomfortable situations.) He cracks up slightly at least once. I’m pretty sure that even if Dave wasn’t in on the act, he’d realise it was an act pretty quickly and probably relished the chance to have a quirky interview.
I can believe it was an act, but I agree with those who said Letterman doesn’t appear to be in on it. And if this appearance was meant to promote some future project of Phoenix’s, well, he’s all but guaranteed I’ll never see it.
On the subject of awkward interviews, this late '70s Peter Gzowski interview with Iggy Pop is probably the best (worst?) I’ve ever seen. I think it’s safe to say that drugs were to blame in that case, though.
I finally watched this (the Huffington Post shortened version) and thought it was really funny. My vote is for hoax, not drugs. It’s almost like he (or maybe they if Affleck is involved) went in with the sole purpose of inspiring Letterman to not pull any punches. If so, I think he achieved that goal. When Letterman makes the Unibomber crack, Phoenix gives a little smirk, almost like he’s thinking “damn, nice dig, now hit me again.” I think it’s exactly what he wanted to have happen. Letterman seemed a little annoyed at first, but then it seemed like he got into it and even enjoyed it.
I rarely, if ever, get to see Letterman anymore, but he’s still sharp as a tack and funny as hell. At the end of the interview, Letterman gets a bit snippy (the gum chewing), but all in all, he handled it wonderfully. Maybe I’ll start staying up later.