Conversation so far:
When did Robert Mitchum start acting?
1942 and 1943.
What was his first film?
His agent got him an interview with Harry Sherman, the producer of Paramount's Hopalong Cassidy western film series which starred William Boyd.
Did he have a big role in that?
Mitchum was hired to play minor villainous roles.
Are there any other interesting aspects about this article?
Mitchum suffered a nervous breakdown (which resulted in temporary blindness), apparently from job-related stress.
He sought work as an actor or extra in films.
What caused the mental breakdown?
CANNOTANSWER.

Question: What other acting did he do?
Self-contained rewrite: besides Paramount's Hopalong Cassidy western film series, what other acting did Robert Mitchum do?