From Barry Miles’ Zappa: A Biography, regarding the Freak Out! sessions:
The session musicians were shocked to find out they had to play from charts: Frank set up a podium in the middle of the studio and showed up wearing a swallow-tailed conductor’s dinnner jacket, a red and yellow striped shirt and an Iron Cross. He whipped out his baton from under his coat and rapped on the music stand. ‘Hey, we’re really going to have to play,’ said one of the cello players. ‘This beatnik has written some music here!’ According to Tom Wilson: ‘Frank gave it the full Toscanini and conducted their asses off.’ Everything went smoothly as the arrangements were relatively easy.
Too easy for some people. There had been a new addition to the line-up before recording: pianist Marc Rebennak left Sonny and Cher to join the Mothers. He showed up at rehearsals with a joint in his mouth and his friend Elliot Ingber signalled to him behind Frank’s back to get rid of it. Rebennak had asssumed from Frank’s appearance thhat he was into drugs. He played on a couple of tracks, including ‘Return of Monster Magnet.’
Rebennak: ‘Everybody in the studio but Frank was wanderinng around high on acid. Frank had written me this part to play, five or six notes on the piano ovver and over—not much different thhan sonny and Cher. In the background, a twenty-voice choir croaked out monster sound effects, something like “Gggrrrrrrhhhhrrr!” When I had had about all I could take, Les McCann walked in and I asked him to hold down my chair, telling him I had to go to the bathroom. I walked out of there and never came back.’ Rebennak later reinvented himself as Dr. John the night Tripper, a musical voodoo shaman.
That’s right. This guy played on “Freak Out!”