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.

Question: Did he have a big role in that?
Self-contained rewrite: Did Robert Mitchum have a big role in Paramount's Hopalong Cassidy western film series?