John Sublett (stage name, John Bubbles) was a tap dancer unable to read music.  He was chosen by Gershwin to perform the role of Sportin’ Life in Porgy and Bess.  However….

Rehearsing as Sporting Life, John Bubbles was a special problem.  He was so laid back as to be often absent when needed.  At one session this was too much for Smallens.  He threw down his baton and shouted to Mamoulian: “I’m sick of this waiting.  We’ll have to throw him out and get somebody else.”  Gershwin, constantly hovering over rehearsals and nervously chewing peanuts, emerged from the darkened stalls.  Frantically mixing up his musical categories, he cried, “You can’t do that! Why, he’s – he’s the black Toscanini!”  Eventually Smallens restored to teaching Bubbles his vocal rhythms by tap dancing them.  Todd Duncan remembered that, during the New York run, “this individual would hold a particular note two beats on Monday night but on Tuesday night he might sustain that same note through six beats.” Gershwin called him “my Bubbles” and forgave him everything on account of his marvellous dancing and his sheer star quality.

Greenberg, Rodney (2008) George Gershwin.  New York: Phaidon Press, p.184