Richard Pryor in a Televised Interview on the Set of Stir Crazy
Evidence: Around 6:29, he launches into a vigorous fit of mock masturbation which, when conducted on national TV, is a pretty good indicator that you're out of your fucking gourd. Pryor also says a few things that might suggest he's on drugs, such as: "I didn't get caught yesterday buying seven pounds of cocaine in front of eight policemen."
Yes, and it appears that you didn't get caught smoking all seven pounds right before this interview.
Drug(s) They Were Probably On: Hard to tell, but we're guessing cocaine.