The first time I saw Eddie Murphy doing Bill Cosby was on the video for the best of Eddie Murphy on SNL. I had to stop it, rewind it, and watch it again before I could go on with the rest of the tape.
Years later, I saw how Eddie's imitation goes when he doesn't have to worry about censorship.
If you just listen and don't watch, you genuinely won't know that you're not listening to Bill Cosby. Or Richard Pryor, for that matter.
And just for the hell of it, here's somebody under 21 lip-synching it.