It takes some kind of charm to play seductive and evil like Mitchum usually did. No one does the combo better, which is why he was cast in Cape Fear and The Night of the Hunter and more recently in Jim Jarmusch's Dead Man. He is regarded as the posterboy of film noir and his arched eyebrow, cleft chin, menacing stature and broken nose look divine on stark black and white film.
But he had his softer side too, sometimes. Well, if not softer, than at least less noir–he was well known as a prankster throughout his life. He also had a very laid back attitude towards fame, as all his quotes will attest to, and an even more laid back style of acting. Maybe it was all that pot he was smoking.