One from "The Maltese Falcon"
Dashiell Hammett's Sam Spade says:
"Don't be sure I'm as crooked as I'm supposed to be."
And a noir line that's also thematic.
"When a man's partner is killed he's supposed to do something about it. It doesn't make any difference what you thought of him. He was your partner, and you're supposed to do something about it."