I'm 42 and I've lived in Boston my whole life. I travel extensively and pay attention to the way people talk. Everywhere. For those of you that are not from here: People from Boston do not talk like the Kennedys. Really. No one except the Kennedys talk like that. OK, William Devane and Martin Sheen do sometimes, but they don't know any better.
Here's the point: Mitchum nails it. He doesn't over-do it (Cliff Claven) and doesn't under do it. Critics claim that Mitchum is good at accents but he really does nail this one - the toughest one: A native Boston accent. That is indicative of the whole movie. Mitchum nails everything. This is his most believable performance. Listen to him in this movie and you could really imagine him as a resident of Quincy. It fits. The bleak, cold hopelessness of the title character's life is played out to its inevitable conclusion. A real classic "not-trying-to-be-film-noir" example of classic film noir.
Signed, The Director's Son (Just Kidding - this is awesome! Watch it!)