I'm continuing a series of writing posts on worldbuilding. Last time, I talked about geography. Now it's time to talk about the people who inhabit the world, since they'll probably have a lot to do with the way your story shapes up. People can be very influenced by the place in which they live. That can have something to do with climate -- you get very different cultures at the equator than you do in harsh northern areas -- or location -- more remote areas are likely to be more homogeneous, while places that are easy to get to will probably end up being more diverse. That diversity doesn't always come without struggle and opposition. If you look at patterns in immigration in port cities in our world, you see initial opposition to newcomers, who at first keep to themselves in enclaves. Gradually, their children assimilate, and the greater culture begins to adopt some of their culture (food, arts, etc.). They become part of the greater culture, which is then resistant to the next wave of newcomers.
What kind of people live in your story setting? Think about their daily lives -- what are their homes like, what foods do they eat, what do they do for fun, what are some of the key industries that might employ them, what kind of arts do they pursue? One tricky area that tends to get left out in fiction is the matter of faith. Most human cultures have had some kind of belief system that explains their origins and the world around them. These belief systems may grow and strengthen, get replaced by something else, morph into something else, or fade away. Even a society that has become more secular has generally made a choice to turn away from old beliefs, and there are people who still hold onto them. Most legal systems have developed from religious laws, and a lot of cultural rules and attitudes have their roots in religious beliefs. Many of the arts were first used as expressions of faith. You get a richer world if you at least consider the question instead of creating a world where this doesn't even come up. You could get a very different society by imagining an old religion with a very different set of beliefs and tracing how those beliefs affect laws and culture. That could change traditional gender roles and views of morality.
Are the people in this area conquerors or the conquered? There are nations that tend to be the aggressors and those that tend to get overrun, either because of their geographic location with no natural barriers or defenses or because of their tendency not to put up a fight. Is the conquered nation currently under occupation or control, or are they autonomous again after being liberated by someone else? History of conquering or being conquered will affect the psyche of a nation and possibly the attitudes of the people living there, as well as their view of outsiders, violence, and the need for a military.
How does the economy work? What is traded? What is produced? Is there currency, or is it a barter economy? Is there any kind of economic control by the government, or is it a free-for-all? What's the distribution of wealth? How do people feel about the distribution of wealth? Is there a class system, and how flexible is it? Can someone move up just by amassing wealth and status, or is birth a major factor? If there is a rigid class system based on birth, how did the upper-class families attain their status, and when?
And is any of this changing, on the verge of changing, or subject to change if outside forces (like events in your story) act on this part of your world? Is someone -- either your hero or your villain -- trying to change it? How stable or precarious is your society? Is it strong enough to survive a disaster, or will it all fall apart if one thing happens? Is there some similar or contrasting society nearby that may affect this society -- enemies that might invade, enemies this society might invade, allies, potential allies? Where does this culture fit into the overall world? Is it better or worse than other cultures?
History is a great place to look for ideas of how a society might develop and what can tear it apart.
Once you've developed all these things about your world, it's time to think about how you'll show them in your story, and that will be the next topic.