Watch Joaquin Phoenix's bizarro behavior on 'David Letterman'!

What was up with Joaquin Phoenix on David Letterman's show Wednesday night?

It's painful -- but painfully funny -- to watch a clearly uncomfortable Phoenix wriggling in his hot seat on Letterman's show.

Letterman starts off by grilling him about his beard, then asks, "Can you tell us about your days with the Unabomber?"

When Joaquin winces, Dave says, "It was just a little joke," adding that it's his hope that after taking some time off ("and I think you are taking some time off tonight") that Joaquin will reconsider and come back to acting.

Joaquin then asks about Dave's thumb, which he explains as a cuticle injury.

And when the audience titters about his hip-hop music career, Joaquin asks Dave, "What do you have them on? What do you gas them up with?"

Dave responds: "The kid in the front is drunk on warm Pepsi."

Oh, no, there's more.

When Joaquin says he would love to come on the show and do his music, Dave says, "You know, that seems unlikely."

When Dave ends the interview by saying, "Joaquin, I'm sorry you couldn't be here tonight," Joaquin perks up and says to the audience, "He's funny. He's a funny dude."

Not as funny as you are, Joaquin. Not as funny as you are.

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