I’ve heard this story told about a number of actors. But the best evidence has it belonging to Edmund Gwenn, the actor who played Kris Kringle in Miracle on 34th Street.
Gwenn had had a long friendship with George Seaton, the director of Miracle on 34th Street. Seaton and Gwenn often argued about which was the more difficult—playing highly dramatic parts or the more comic ones. Seaton insisted that the height of the player’s art was tragedy, while Gwenn, who had toured England as a journeyman rep actor playing Shakespearean tragedy and comedy, maintained strongly that it was more difficult to make an audience laugh.
Gwenn was considerably older than Seaton, and in the late 1950s fell into ill health at the Hollywood Motion Picture Actors’ Home. Seaton would visit him from time to time, but on one particular day, Gwenn was in his bed, looking very tired and ill.
Seaton, sad to see his old friend so sick, said, “This must be very, very difficult for you.”
And Gwenn replied weakly, “No, no, no. You still don’t understand, George. Dying is easy. Comedy is hard.”
And with that, the veteran actor died.
(With thanks to http://quoteinvestigator.com/2010/10/26/comedy-is-hard/)