A 1982 Village Voice profile explores why Mitchum was popular with both Hollywood money-men and movie-goers. For the former, it’s because he did his own stunts — “directors [get] two performances for the price of one.” That might be surprising for a man reputed for his indifference; the title of his biography (written by Lee Server) was even, “Baby I Don’t Care.” Clearly courage came with that indifference too.
What about audiences though? The profile’s writer, Carrie Rickey, argues that it all comes down that old adage: Women wanted to be with him, men wanted to be him.
“Of all the postwar actors — Kirk Douglas, Burt Lancaster, Richard Widmark — only Mitchum immediately figured out how to be a man’s man and a woman’s man at the same time. His Kind of Woman could just as well be titled Her Kind of Man.”
Sure enough, many of Mitchum’s genre pictures — “Out of the Past,” “His Kind of Woman,” “Heaven Knows Mr. Allison” — are romantic and helped along by his chemistry with leading ladies from Jane Russell to Deborah Kerr. At the same time, the type of men whom Mitchum played— war heroes, cowboys, private detectives, and charming rogues — meant he was never pigeonholed as a romantic lead no matter how many women he made swoon. When he wasn’t in need of his seductive power, he could switch it off for the cool aloofness that made him so admired by men. Even if Powell and Cady are too slimy to be charismatic, and Eddie Coyle too pathetic, Mitchum’s charm never totally disappeared no matter the role.
Mitchum’s brand of masculinity — being strong enough to show weakness — is a flavor that remains all too rare even today.
Source : https://www.slashfilm.com/956080/the-reason-robert-mitchum-was-in-such-high-demand-as-an-actor/