Dialogue is particularly tricky in historical fiction because this is when the characters, who by definition live in a different age and society, come most to life. Because characters are supposed to be embedded in their own time, it is tempting to put antiquated speech into their mouths in order make them sound like they are "historical." The problem with that is that most authors don't have a good grasp on how people actually spoke in times past, and the problems get greater the farther back in time a novel is set. It is useful therefore to segment the discussion into two parts: Novels set in the ancient and medieval eras, and novels set in the last couple of centuries.
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