Orson Welles was actually battling depression at the time of filming, and had to be coached through his lines by director John Huston
Playing Against Type: The primary reason that the 1956 version wasn't well-receieved initially was that audiences weren't comfortable seeing Gregory Peck as a villainous character (albeit an Anti-Villain). Patrick Stewart and William Hurt faced no such problems.
What Could Have Been: William Holden and Robert Mitchum were both pitched as possible alternatives to play Ahab, as they didn't think Gregory Peck was capable of playing a villainous character. Interestingly enough, Peck himself thought that director John Huston would have made the perfect Ahab.