My wife and I just watched the movie No Country for Old Men, which is set in West Texas ca. 1980, and I couldn't help feeling that the screenwriter was laying it on a little thick with the regional dialect, to the point where it seemed distracting and felt fake.
If we single out a particular time and place like this, and people with a certain background and level of education, can we find data about how people would really speak? E.g., in the movie, Carla Jean Moss says "knowed" rather than "knew." Is there some way to tell whether this is realistic or not? Another example from the movie is that everyone replies to negative questions by saying "yes," and then the negation, as in "Yes, we have no bananas."
It seems like it would be difficult to get accurate data. For one thing, people will probably adjust their register of speech when they're being interviewed by someone from outside the area. And people differ a lot, e.g., my wife and her sister both grew up in Buffalo, but my wife pronounces "man" with a generic newscaster's accent, but her sister says it with a very strong regional "a."
Is this kind of data only available in paywalled academic publications, as case studies for specific times and places? Or are there books like "Atlas of 20th century American speech," or "A writer's guide to American dialects?"