The Onion AV Club interview with Frank Miller has some really good bits. One of the more interesting (IMO):
O: One particularly interesting thing about your work is the language your characters use, especially in the two Dark Knight series and the Martha Washington books. How do you develop a realistic-sounding slang patois?
FM: The one in Dark Knight is the one I can really answer. In the other books, it’s just stuff I overheard or made up. But in Dark Knight, it all has to do with the town where [wife and colorist Lynn Varley] grew up. Her brothers—Don and Rob, by the way—were part of a bunch of kids who talked in this very peculiar manner. Whenever I heard it, I’d just go nuts, because I loved it. It was this very sarcastic mode of speech. One time I was in Michigan visiting the family, and I sat the two of them down and had them do it into a tape recorder, and I went back and studied it. Then I wrote my stories, and I would always show those parts to Lynn before I got it lettered, and she’d tell me where I got it wrong.
O: So the gang members Don and Rob in the two Dark Knight series are actually speaking a legitimate street slang?
FM: The way some kids used to talk back in the ’60s in one suburb of Detroit.