Although not a member of the original Mercury Theatre, Duffield was Welles' chief collaborator for The Mercury Summer Theatre show, writing many of the shows either with Welles or by himself, and acting in several of them. He clearly wrote the MOBY DICK show, which may be the only surviving document of Welles' playing Captain Ahab. In the Welles archive at the Lilly Library there is a huge script for Moby Dick - credited to Duffield - which the 30-minute radio show may have been a condensation of. Welles later planned a cantata of Moby Dick with music by Arthur Honnegger and the libretto by Duffield. Nothing ever came of it.
Duffield also played several roles in Welles' disastrous 1945 theatrical extravaganza AROUND THE WORLD (in the theatre section of Wellesnet, there's a program for Around the World which lists a brief biography). He also had a brief but significant cameo in Welles' film of MACBETH, playing the first murderer who Macbeth hires to kill Banquo (William "Thompson the Reporter" Alland played the other murderer). Duffield's part was diminished for the 86-minute theatrical release of Macbeth, on the order of Republic execs, who disliked his performance. His full cameo can now be seen in the restored version.
Duffield later became a noted writer for TV, succesfully adapting several classic works for the small screen, including Alice in Wnderland, The Hobbit, and probably his most famous adaptation- of Shirley Jackson's The Lottery.