Charleston is different than Miami, which is different than New Orleans, Houston, Albuquerque ... etc. And big cities are different than smaller towns.
Heck, Charleston is different than COLUMBIA.
Okay, seriously, though.

I've lived in nine states, and generalities irk the heck out of me. (While in jr. high in Texas, a girl on the bus found out I'd been born in NJ. "What's it like," she said with big eyes, "Up North??" As if Up North were a place inhabited by two-headed aliens.) But it's equally true that culture can vary a lot from place to place, even within the same state (and yes, I'm serious about what I said above, too!) What it boils down to, though, I think is this: Everywhere, people are nice. And everywhere, people have the same basic feelings. But their ways of being "nice" and their ways of sharing those feelings are different. In Charleston, the way to be "nice" and the proper way to be polite in a grocery store is to meet eyes with whoever you pass and say hello. In Michigan, the proper way to behave in a grocery store is to stay out of people's way, and don't make them nervous by staring at them like a weirdo and making striking up unexpected and possibly unwanted conversation. Both people think they're being "nice."
Another reason people have cultural differences is that the specifics of their environment (weather, what kind of jobs people do) can affect how they approach different kinds of tasks. The fishing industry on an island might bleed into the culture, just as the long winter might, or farming, or working in a very dense area where there is little privacy unless you *decide* to ignore what's staring you in your face. (Which may come across as unfriendly, but in a way, it's allowing someone else to have a little privacy they wouldn't have otherwise if you crowd 8 million people onto an island together.)
I agree with Ani Louse--the best way to make something feel real is to make your characters specific *people.* That way, their quirks will feel authentic to them, even if they don't represent the entire species.