One of the things I try to keep in mind when I write is regionalisms. This is word choice/slang that is common in one area of the country and unfamiliar to other people. This isn't something that I'm always thinking of, but I do try to have it in the back of my mind. Authors have leeway, but only to a degree IMO.
As an example, writers who do the Fargo movie speech for someone from Minneapolis make me insane. I'm from Minneapolis and it's not something I hear often. The people who do talk that way in the Cities tend to be older and tend to have grown up in outstate Minnesota. (Outstate is anything other than the Twin Cities or its suburbs.) So if I read a book and the author has their 20-something secondary character who grew up in Minneapolis talk like Fargo, it takes me out of the book.
You betcha. Uff-da, uff da. Yeah, right. If someone is talking like that, they're parodying how others think we speak.
That's not to say Minneapolis doesn't have regionalisms. The biggest one is how we tend not to finish sentences. What you'll hear is something like: Do you want to go with? To me, this makes complete sense. To a friend of mine that grew up in North Dakota and moved to the cities, it makes her nuts. Go with? Go with who? Go where? To her, she needs to hear Do you want to go with us to the mall this afternoon? for it to make sense. To someone who grew up in Minneapolis, we infer the rest of the sentence because we're talking about going to the mall after work, what else/who else could we mean? :-)
Traveling a lot helps with regional differences, but so does hanging around online with people from different areas of the country. I try to pay attention to how someone from the west coast words something versus someone from Texas versus someone from Alabama.
When I wrote Eternal Nights with my hero who grew up in Ft. Worth, Texas, I had a friend who grew up in the South correct my southern. :-) Not that I did too badly on that. I pick up the speech of others pretty easily and I even pick up words my characters use all the time even if I didn't use them before I wrote a particular story. Anyway, my biggest mess up that she corrected was how I used the word fixin'. I used it as in the future some time, she told me that fixin' is the immediate, like in minutes, future.
In Edge of Dawn, my hero, Logan, grew up in the Chicago area and my heroine, Shona, in Seattle. So when Logan refers to the freeway, he calls it the expressway. Nearly all my cousins grew up in Chicago or the surrounding area and they always call it the expressway. I'm guessing since there are so many tolls, that you can't really use the word free in relation to their interstates. :-) But Shona calls it the freeway. And I did ask on Twitter because I wasn't 100% sure on Seattle. I've only been there twice.
I also think there's differences in speech between city/suburban people and people who grew up in rural areas or small towns. I picked up some of that when I was in college out in Morris, MN with a lot of small town/rural kids.
So IMO I believe that writers should be aware of where their characters grew up and how people from that area tend to speak. Not that it should be copied exactly because heaven knows it gets extremely annoying to read authors who feel the need to write "dialect" in their dialogue, but using words those characters would use like calling the freeway an expressway is important to writing real people. Again, JMO.
