An excellent interview with Deadwood producer David Milch by Allen Barra. It includes this:
Don’t you think it had more to do with the idea that that language was used in the context of a Western? That people weren’t used to hearing those words used in a setting that Gary Cooper and John Wayne once inhabited? I mean, for older viewers at least, the saloon talk sounds a heck of a lot saltier than anything Matt Dillon and Miss Kitty said.
That probably has a lot to do with it, but if you’re going to talk about language, I wish more people had noticed the overall language, the rhythms of period speech that we tried so hard to re-create, and the richness of the imagery. Profanity, I’ve come to believe, was the lingua franca of the time and place, which is to say that anyone, no matter what his or her background, could connect with almost anyone else on the frontier through the use of profanity. But there’s so much more to the dialogue than just the profanity. The language of the characters in the show is never generic, and everyone’s is different. They come from different backgrounds, different parts of the country, and they all express themselves a little differently.
That’s one of the things people like about the show, that after they’ve watched for a while, they can instantly identify each character by the quirkiness of his or her speech. These are people, you know, who all grew up long before the age of electronic media, when regional speech patterns began to lose their distinctiveness. Many of them might have been illiterate, but they knew the King James Bible and Shakespeare, and that’s what shaped the way they thought and the way they expressed themselves.