The whole script is a primer in hard-boiled dialogue. Interestingly, according to Ebert, the person credited with the script didn't write any of it!
But the critic Jeff Schwager read all versions of the screenplay for a 1990 Film Comment article, and writes me: "Mainwaring's script was not very good, and in one draft featured awful voice-over narration by the deaf-mute. Cain's script was a total rewrite and even worse; it was totally discarded. The great dialogue was actually the work of Frank Fenton, a B-movie writer whose best known credit was John Ford's 'Wings of Eagles.'"
I'm definitely going to check it out, but if it's better than Blood Simple I'll eat my hat.