The hallmark of any decently habitable world is that it has its own ecosystem. Any ecosystem worth its salt should be self-sustaining. But when you’re building it from scratch, it helps to know which elements act as catalyst engines…and which will stall your story.
People Are People
The most obvious catalyst for the ecosystem is to throw in interesting people. A world populated by interesting people–with their own needs, goals, wants, and flaws–churns on under its own power. Now, your story can’t possibly follow along with all eight million stories in the Naked City, but the fact that those stories are happening gives your world a life and an inertial speed all its own. Even “walk-on” characters need motivators that determine whether or not they help or hinder the heroes with whatever conflict units that engage them.
There’s (Conflict) Gold In Them Thar Hills
Never underestimate the power of what’s under your (characters’) feet. “Man vs. Wild” isn’t just the name of a popular survivalist show, it’s also one of the core, basic conflicts of all stories–the others being “Man vs. Man” and “Man vs. Self” or, as Kurt Vonnegut put it, “Man in a hole.” That hole right there is your main character’s adversary.
Time and Tide…
Time flies when you’re having fun, and when your world is chugging along with people who want things, and settings that provide obstacles to them getting those things, it’s not doing so in a vacuum. That waitress may never win the lottery, and she may have all the time in the world to play, but sooner, rather than later, her tips are going to run out for her rent/kidney dialysis. The passage of time makes her situation (the need for money) turn critical with respect to her motivation (home/kidneys), and forces her into more drastic action (getting up her courage to ask for a raise/rob the till). Worldbuilding inertia slows when characters can continue on as they have been without change in their consequences or circumstances. If your waitress merrily keeps buying lottery tickets without a change in either the need to win or the drain in her finances, then she’s not a catalyst engine–she’s moving scenery.
Keeping The Engine Oiled
When you keep things moving in your worldbuilding, you not only create a denser, more populated environment in which to showcase your heroes, you also create a world that has enough depth and definition to support a living, breathing story. Any good playwright knows that if you’re going to have flying characters, you’d better have a decent theater from which to hang them. And if you do pull it off, you’ve not only used your actors, sets, and script, you’ve also made the theater itself, and even the audience an important and dynamic part of your story. And if you can pull in your audience that way, you’re moving in the right direction.