Robix points out this feature on Chris Elliott, in which Charlie gets a brief mention, re: his time on Get A Life:

AVC: What's Charlie Kaufman like to work with?
CE: [Laughs.] At the time, he was very quiet, you know? He was—if it was like Your Show Of Shows, he would have been—I guess Neil Simon was the one who used to whisper ideas to Mel Brooks? Charlie Kaufman, if he had anybody who would listen to him, he was that. He was very shy and quiet, but always wrote funny stuff.