Why did the Coen brothers decide to follow the diction of the original novel so closely? Did folks in this time period really speak this way?
When asked why they chose to follow the diction of the book and lack of contractions Ethan Coen said
We’ve been told that the language and all that formality is faithful to how people talked in the period.
As to whether or not people actually talked like that Mark Liberman of the University of Pennsylvania Dept. of Linguistics writes:
I know that that informal American speech in the 1870s was far from contractionless, and in fact I suspect that it had roughly the same proportion of contractions as it does today. Therefore, what Portis (and the Coens?) did was either false archaism or poetic truth — or both.