Gilbert, of course. This is what purports to be his first appearance on Late Night with Conan O'Brien early in the show's run. They were doing a Halloween theme I believe and the premise of the first part is that Dick Cavett is pretending to be Gilbert in a Dick Cavett costume. Gilbert is off-stage live-dubbing the voice and one gets the feeling that Mr. Cavett was not thrilled with some of the words coming out of "his" mouth.
I believe the previous guest seated on the couch is Maureen McCormick of The Brady Bunch. She does not seem to know what to make of the crazy person who comes out and sits to her left.
Gilbert was hysterical on a lot of shows like this and I once asked a writer for one of them why they didn't have him on more often. The answer was roughly, "He upstages the host and doesn't give that host a chance to be funny." A host, of course, wants his comedy guests to get laughs but some of them don't like the idea of not getting plenty of laughs themselves during a segment. They don't like when this happens…