Life brings a convergence: Fave blog LanguageLog.com posts about portraying historic and regional dialects in fictional dialog. http://languagelog.ldc.upenn.edu/nll/?p=2554
Language Log is a group of linguists who blog on a wide variety of topics, usually in a way accessible to any educated reader, not just to linguists. But they rarely blog about writing fiction, so this was a fun read.
It's hard to write characters producing realistic-sounding dialog, contemporary or historic, without annoying the reader. For example, I have a young college student speaking to Professor Rousseau. Now, I happen to know that many 20-year olds out there use the word "like" as a high proportion of their total word counts. I wanted to get that across in her dialog. But if you have to read more than a line or two, you might be tempted to put that book down and never pick it up again. It's as irritating as hearing it in person. So I used frequent "like"s in the first line or two and then let them subside.
Because my protagonist is a linguist, she often notices how people around her say things, and uses her ear for that to identify a suspect in an overheard conversation. It's an interesting challenge to slip in language-related clues wherever I can without making it obvious.