Writers around the globe owe a sort of cultural debt to the rest of the world. Without the rich diversity of cultures on planet earth, would we have the same rich tapestry of characters in our novels? Almost all of our fictional worlds are peopled with cultures that resemble real ones we know, some are just less familiar to us.
I'm no different. I didn't even try to create a fictional culture for my current novel series. It's based on Creek (Muskogee) folklore, and set in rural Arkansas. That's pretty familiar to us. However, even the fantasy worlds are often borrowing from real cultures.
If I described a people group who rode horses with highly arched necks, carried curved swords at the hips and wore billowy pants that cuffed tightly at the ankles, what do you imagine? Dark skin and men with beards? Without being told, you probably immediately imagine a middle eastern culture of yesteryear. Maybe Persians? You will likely assume certain belief systems and cultural norms for this people without the writer saying so.
If I described, on the other hand, a group of people that lined their eyes in dark black kohl and thrived in a desert climate, you probably are going to think of the ancient Egyptians.
Think about a book that you love. Maybe the women wear regency-style dresses, harkening back to a bygone English era. Maybe the men fight in full plate armor, bringing to mind medieval Europe. Maybe they wear long robes, or togas, and we are immediately going to think of ancient Greece or Rome.
You can try to come up with a culture all your own. It's possible. It's also hard. Without realizing it, we build up cultures around ideas that we are already familiar with.
And if you are going to have good story, with compelling characters, you have to think about culture. Borrow from existing cultures, base the story in real cultures, make up one entirely your own, whichever you decide, you absolutely must have them.
How your characters dress, what they eat, even how they swear will be largely determined by culture. You can't write a story without it. Every decision your character makes will likely be based in his/her cultural values and beliefs.
Today, I would say to you first: Don't neglect the culture of your characters.
Second: Handle other people's culture with respect.
Whatever culture you choose to borrow from (or base totally in), try to remember that it is real to someone. In my novels, I am taking from a real, living culture. There are people now who still participate in the Creek (Muskogee) culture. They are alive, they still hold some of these beliefs, and the folklore I'm borrowing from is still being shared among them. There are folks who still speak their language, and who are still as engaged as ever in it. Is it fair then, for me to write about it, represent it to the world, without at least a baseline of research to represent these people accurately?
Now, at this point, so many things have been written with vague references to medieval Europe that we have a hard time parsing out fact from fiction. Part of why I was motivated to try to represent a particular Native American tribe as accurately as I could was because "Native Americans" are in danger of the same misrepresentation. How many times have you seen a Native American represented that was wearing feathers or a full headdress, riding a horse across a field and living in a teepee? That is a good representation of a plains tribe, but there were countless others that lived very different lifestyles.
Even if you select a "dead" culture that no one practices, I would still encourage you to do a modicum of research. Why? Often that little bit of digging will unearth information you can use to make your characters and stories that much more interesting. You don't have to be an expert on ancient Greece. Try to give thought to why a culture did some of the things they did. Then you can decide if your fictional culture would come to the same conclusion.
At the end of the day, we have to remember that writers are writers and not necessarily historians or anthropologists. Fiction is meant to be, well, fictional. We can't get everything right, that's not even the point. However, writing about an existing culture without any thought to the people we represent is lazy and reckless.
People are people. We must try to respect each other, even when it comes to our fictional avatars. And fellow writer, I can tell you that you won't regret the work you put into it. It will not only make your writing better, it will make you better.