How do we define one people from another? Is it by appearance? Then what about the USA, or South Africa, or any other cosmopolitan nation? All right, then it must be by language? What about Switzerland, where they speak German, French, and Italian, or China, where dialects can differ so much that you may not be able to understand someone using a foreign dialect of the language?
It's by the art and culture that we are defined, and that's how it should be in writing, too. Go ahead and pull out your map, with all of those precisely designated countries you hand-rendered, the rivers you tracked down from the mountains, and the carefully drawn microtrees to designate a forested area. Now, what is it that makes one little glob different from the one next to it? If you say anything but art and culture, you're not necessarily wrong, but that should be in there, right up at the top of the list.
Now, I can't rightly separate art from culture, as they both shift and change with one another. Be it your country's food, drink, dance, theater, government, architecture, religion, literature, or whatever else, they should have a unique world view. Do you have to know all of that? Yes. Even if you only really deal with one character form that country? Yes. A country's art and culture will affect its people in such a way that it becomes inextricable from their being. Don't believe me?
Try to imagine how our country would be different without the Beatles, or Dr. Seuss, or the Rolling Stones, or Bob Dylan. It seems weird, but think about every ripple that never would have formed. You really can't, and that's the key thing. Its easy enough to track their influences on culture from the onset, but if you try and remove them, its a totally unpredictable mass of nothingness.
That's how your cultures need to be portrayed, or they probably won't come off as the most clear image that they could. Now, I'm not saying I'm perfectly amazing at this, since I'm not, but the first step to fixing an issue is seeing it, and every writer out there should be behind me on that point, yes?
What, however, influences your art and culture. While it's true that sometimes things are just flashes of inspiration from the gods, particularly on the artistic side of the equation, for realism's sake, one should trace them back to something greater that the people can't really control. Like, if you live in a tundra without heating or other modern conveniences, are you really going to have a face studded with piercings? No, because your whole damn face would freeze, turn black, and shrivel into a frostbitten husk. All right, that might be an exaggeration, but it would be highly uncomfortable and you know it. Look at a desert culture—your first instinct is to have them wearing light, skimpy clothing to escape the searing heat, but when you look at actual desert cultures through history, they wear more clothing than anyone else, because they have something more pressing to worry about than heat—sunburn. Heat may be uncomfortable, but sunburns can be lethal.
Also, why are they divided here? Why isn't this just one giant country like China or Russia? Is it an old religious dispute, governmental disarray, a river, a road? What force is so powerful to keep two countries apart, and is it a point of contention?
Where did their culture come from? Is it wholly original or did they, like many countries on Earth, borrow, assimilate, and steal various important parts of their culture? Are they the Chinese, originating the written characters, or are they Japan, taking the Chinese characters and changing their meanings?
There's so much more I could go into on this subject—hell, books could be written about this subject, if someone had the time—but I'll leave it at that. It's enough to get us thinking, right?