Brian Eno, he of the Oblique Strategies, has been sharing new strategies on Twitter for the last little while. Here's one he posted over the weekend that completely sums up my current approach to writing in general, and building worlds and characters in particular.
Start simple, get complicated, then re-simplifyThat, right there, is the essence of making an interesting character or setting or world, I think. Start with a simple idea, then explore all the possible ramifications and permutations and iterations, then throw out everything that isn't essential. I do all of this at the planning stages, long before writing, when I'm figuring out who the characters are, or where their story takes place. Often the end result looks nothing at all like the input, and the readers never sees the complications in the middle, but I've found that this process is often key in taking something that's just a notion, a bare idea, and turning it into something that works.