Irwin Charone (J.P. Pritchfield) appeared 5 times in the series, four of them as one of Darrin's clients. He also once played Lester Dracula, Lily's brother and a classy if not financially irresponsible werewolf, on Herman's Rival (1964).
Leonardo and Michelangelo were not of the same generation. Leonardo was 23 when Michelangelo was born. Leonardo died in 1519, but Michelangelo went on creating for another 45 years.