Giving my characters each their own distinct, believable voices is both one of my favorite parts of writing, and one of the greatest challenges.
Since my novels are based over two hundred years ago, there are obvious problems of dialect and colloquialisms to overcome – all too many times, an early reader has caught my characters saying, “Okay,” because it’s a common interjection in my own speech. It is, of course, utterly anachronistic, and must be brutally expunged.
Spoken English in the Colonies, of course, would have been virtually unintelligible to the modern ear. Vowel shifts (as revealed by some contemporary poetry that no longer rhymes in modern English), differences in cadence and the gap between modern and Colonial vocabularies would all do their part to challenge comprehension.
More subtly, the Colonists at the time of the Revolution were of widely varying histories, ranging from the long-established to relatively recent immigrants, and so they had differences of linguistic background that I needed to account for.
In one novel, a major character originated as a lower-class Frenchman; I voiced his dialog as I wrote it in an outrageous Inspector Clouseau accent, which seems silly in retrospect, but it helped me to keep his words reasonably clearly influenced by his history.
In another, a character was a wealthy Southern-colony landowner; his speech patterns were deeply offensive to the modern ear, but I could see no way around them without doing violence to the way that a man of his position in that time and place would have spoken.
And the Quakers… I still have a headache from trying to accurately render the decidedly archaic usage of the English second person singular verb and pronoun forms, without straying into the disrespectful ground of apparent parody.
Writing believable dialog for any character outside of autobiography (and even there, to be honest) is a substantial challenge. Doing so with the linguistic and cultural challenges of writing historical fiction is just downright fun.