World Building is one of those things that we all have to do for our games and can be a key difference between a well thought out, thriving, and living setting and a setting that is just 'that place where stuff happens.' Even when we are working with a pre-made setting such as the Forgotten Realms, Greyhawk, the world of Shadowrun, or the war torn 41st millenium of 40k we end up with world building to do. It's a topic I've touched on here before, but with it back in my mind as I start up some new projects I figured it would be worth touching on again. Shall we?
Purpose For World
The first thing you want to do with any world building project is identify what you are world building for. If you are building for a game then you want to approach things a little differently than if you are approaching for a story. Why? Because the game is going to use the world differently from a story and as such has different important pieces. For example, the rulers of an area may not be important in a game but they will likely come up in a story as something important. In fact a lot of setting details that don't relate to "stuff we're doing" or "rewards we get" may not show up in a game simply because the PCs are trying to get in on the action. In a story this is not always the case.
To break it down simply: in general if you are approaching world building for a game than you want to prioritize elements of the world that will generate events and activities for the players to do or respond to. If you are approaching world building for a story, than you want to prioritize elements that will help give the world character, meaning, and story. The two can overlap, or even be the same thing, but you want to know why you are building it and keep that in mind going forward.
An Example of Approach
To go over this lets take a look at a character that could be in the world. For easiness's sake we'll use a fantasy trope: the King's Advisor.
Now, for both we start with the same approach: How does the King's Advisor feel about the PCs/Protagonist and what does he do?
Now, for a game approach our first focus should be on the events the character will generate. For example, if he is the contact to give out missions than we should know who he gives them out for and in what way. Where will he send the players, what is his angle in those missions, and things pertaining to the event(s) of the missions/quests that he gives out.
However, for a story approach we want a different thing first. It is less important the events he sets off (unless they're key to the main plot) and it is more important in how he does it. Is he arrogant? Is he rude? Does he look down on those who do the missions he needs done?
For the best experience both should be done, definitely, but for the game you want the mechanics of the game and plot to work. You need those to flow, you can build the character around that. For a story, aside from the main structure of the main plot (which should be decided outside of world building) you want the character first and let that determine the actions and mechanics. You can do it the otherway for both, but it can lead to complications.
Leave Room For Growth
The last (and second!) point I want to talk about is important for both types of development. You want to leave yourself room to grow. No matter what, your PCs - or protagonists - are going to do things that you didn't expect. You're going to need something in the world that you never considered. If you sealed all the hatches and filled every nook of the world you don't have room for that, and that is bad. This is why even with a set world you need to do world building. Your story will push at the boundaries. Your game will set fire to the status quo. it isn't a matter of 'if' it is a matter of 'at what point' and being ready for it means that you need to have space for the story/game to stretch out into.
Because of this, try to only define things that are definitely going to be important or do big things. Leave the rest more vague. Let it be amorphous and take shape as it comes into camera. Don't force it into a shape before you know everything that will happen with it because I can guarantee you that if you do it'll fight back at some point. So, just give it room. Everyone will be happier that way.