He was unemployed after the war, could't find a job he wanted to do, his mother told him he should try that acting thing.
Lots of B movies and TV, but the first serious movie he actually stared in, "Marty" in 1955, he wins an Oscar for Best Actor. Everyone should see "Marty", then compare his acting in the earlier "From Here To Eternity", and later films, "The Wild Bunch" comes to mind. The man could do anything, unfortunately, casting wise, as Smokey points out, "Not just another pretty face".
