My reasons for looking at dialogue in a different way were mainly because I was heartily tired of reading what I have taken to calling the Berlitz phrase-book approach to dialogue and character-thought. In the phrase-book approach all language is modern, except when specific words are inserted. Sometimes words from entirely the wrong language are used: Modern French instead of Old or Middle French for the Middle Ages, for instance. Get me after a drink or two and I’ll tell you which writers in particular get their languages wrong, but otherwise I shall mutter their names to myself, unhappily.
That is from this fascinating blog post: “Dialogue in Novels — a Medieval Experiment by Gillian Polack.” For those interested in how to balance the historical real with the contemporary reality — that is what your intended reader will make of it —this is very good stuff.