Disclaimer: I'm a fool for the Coen brothers, so this isn't a "review"--I mean, I like The Hudsucker Proxy.
The Man Who Wasn't There is one of the Coen brothers' period pieces--like almost half their films--and they work hard to reproduce the essence of film noir: claustrophobic immobility in the face of a random disaster that is spurred by one reckless act. (I think that covers it.) In fact, the mood hones so closely to the quiet desperation of noir, and the details of its late-'40s milieu are so closely observed that, like The Hudsucker Proxy, The Man Who Wasn't There threatens to become parody. But, like their more recent A Serious Man, an air of detachment rescues the film from self-conscious homage/pastiche.
As so often, the Coens are very lucky/smart in their casting. Billy Bob Thornton's granite-faced barber strikes just the right note of ambiguity: he is a victim but also a perpetrator, wronged but also perilously wrong. Not since Sling Blade--OK, maybe Friday Night Lights--has Thornton delivered such a controlled performance. (And kudos to the Brothers for recognizing the Boris Karloff who hides behind Thornton's face, the sad monster you both pity and scorn.) As for the rest of the cast: I'll let you discover the pleasures of their performances, everyone infected with Coen-commitment, as though they'd been rehearsing for these roles for a long time.
Yes, it's an unhappy movie moving haltingly about in a dim and uncertain space--the flying saucer scene remains one of the Coens' great elusive (allusive?) moments. But, if you want to see what may be the best adaptation of an imaginary James M. Cain novel, The Man Who Wasn't There should be--there, that is, in your Instant Play Queue.