The funny thing about regionalisms is that you probably don’t realise they’re specific to your area until you go somewhere else or speak with people from another place. Suddenly they think you’re a hick for using certain words or expressions, or can’t understand what something means.
While I’m a proud native Pittsburgher, I’ve lived most of my life by Upstate New York and so don’t have a vocabulary peppered with Pittsburghese. However, there are still a number of expressions I’ve only relatively recently found out are peculiar to the Pittsburgh area. For example, we often leave out the “to be” infinitive in conjunction with the verbs “like,” “need,” and “want.” For example, “The plants need watered,” “The baby wants fed,” and “The dog likes played with.” We also say things along the lines of “Where is he at?”
Sometimes I find expressions like these creeping into my writing, though other times I manage to leave them out. If I’m not writing a Pittsburghese character, it probably wouldn’t be correct to leave them in. And from having so many Orthodox friends, I’ve picked up the grammatically incorrect habit of using “by” as an all-purpose preposition. Examples of how this quasi-Yiddishism (which is also found in Slavic languages) is used:
“We ate by the rabbi’s family last Shabbos.”
“I’m being by my friend for the holidays.”
“She was really acting up by naptime.”
“We met by the bar mitzvah.”
“He stayed by the Kagans and was by shul for the baby naming.”
“We studied by Boston University for graduate school.”
I know this isn’t proper English, and that I shouldn’t use this kind of expression for a character whose native language isn’t Yiddish, Polish, Russian, etc. It’s the same way I try to pronounce “creek” properly among my New York friends, though since there’s a Sand Creek Road here, sometimes I find myself using the “crick” pronunciation I picked up from my Pennsylvania roots.
If your characters live in New York City (unless they’re recent transplants or don’t have deep roots in the city), you’ll want to make sure they say, for example, “on line” instead of “in line.” It doesn’t matter if you think it’s grammatically incorrect or sounds weird; it’s just how people in the city talk. It’s the same way you shouldn’t project your own regionalisms onto characters from different areas.
It’s one thing for me, as the third-person omniscient narrator, to refer to dippy eggs or pop, but a character from Florida or Maine wouldn’t have those terms in his or her vocabulary. By the same token, my characters say “grandfather,” “grandpa,” or “granddad.” I grew up knowing the term “grandpap,” but I never heard anyone in New York using that term, so I kind of instinctively sensed I should avoid it in writing. Only in adulthood did I become aware of the fact that it’s considered hickish outside of Southwestern Pennsylvania and West Virginia.
Also, consider your characters’ socioeconomic class in addition to their geographical background. A working-class New Yorker will probably say things like “outta,” “wanna,” “gotta,” “gonna,” and “shoulda,” whereas an upper-class blueblood from a tony Rhode Island suburb wouldn’t dream of using slangy contractions. People with education and money also usually wouldn’t say things like “ain’t” or use double negatives and improperly conjugated verbs.
Oh, and I might be from Pittsburgh, but you can take me out back and shoot me if I ever say “yinz/younz/yinzes/younzes.” I didn’t even know this expression existed till my junior year of high school, but I was too embarrassed to ask the teacher what it meant. I’m fine with my New York City characters saying “yous” and “yous guys,” but the word “yinz” is like nails on a chalkboard. On that note, please don’t phonetically render an accent, unless it’s done judiciously or for a substantial reason. Just introduce a character by saying s/he has a strong Brooklyn, Southern, Irish, German, etc. accent, and use some regionalisms or foreign words for a sprinkle of character.