Conversation so far:
When did Robert Mitchum start acting?
Robert Mitchum starting acting in 1942
What was Robert Mitchum's first film?
His first foray into the genre film noir in the 1944 B-movie When Strangers Marry, about newlyweds and a New York City serial killer.
Did Robert Mitchum have a big role in that?
supporting role
Are there any other interesting aspects about this Robert Mitchum article?
In Where Danger Lives, Mitchum played a doctor who comes between a mentally unbalanced Faith Domergue and cuckolded Claude Rains.
What caused the mental breakdown?

Question: What other acting did he do?
Self-contained rewrite: What other acting did Robert Mitchum do?