When it comes down to it, I'm more of a character writer than an idea, plot, or world writer; however, in a world like my DragonEye, PI, world, the two are very interdependent. The characters build the world, yet the world defines the characters, and the plot and ideas tell me what characters need to make an appearance.
For example, in Magic, Mensa and Mayhem, I needed someone to do some discreet searching--and if they could botch the job in a funny way, all the better. We all know the legend of the brownies--the Shoemaker and the Elves, the brownies who are supposed to clean your house for milk, there are plenty of legends, stories, even filk songs. I decided it'd be fun to have the brownies search and, as long as they were in the area, do what comes natural and clean, fix and finish things. Sometimes, that's a big help; sometimes, it's an annoyance, like when they re-arrange things to their logic, not yours. (Kind of like when your mother offers to do your dishes and puts things away according to where she'd put them.) Sometimes, it's a pain, like when they finish your crossword puzzle book. Sometimes, as artiste Melchoir Rawlings discovered, they decide for you your work isn't finished. ("My art! My beautiful art. Oh, it's too much to process! Deep breaths, deep breaths!")
So now I have good-intentioned, havoc-wrecking, independent-minded brownies loose with people who'd love to capture, hire or exterminate them. I have to keep them safe!
Here's where the worldbuilding comes in. I started with an idea I got from someone's terrific effort of explaining Santa Claus's Christmas deliveries: Santa operates in a state of quantum flux that allows him to be in constant motion. The reason you can't see him on Christmas Eve, is the idea that you can either know his position or his motion; so, if you see him, you know where he is; he can no longer be in motion and the whole quantum flux things ends with you getting coal in your stocking.
That's a pretty bad explanation of something I read several years ago, but it was enough to apply to the brownies: They operate in our dimension in a state of flux, so you can only know the things they've done. You can't observe what they are doing directly. Further, if you know you are seeing a brownie in action, that belief plays into it, too. Based on that, I set up the rules of the brownie world.
And then the story demanded that I find a way around them. So I decided to play heavily on the uncertainty principle and the idea that you can observe the effects as long as you don't observe them. And I came up with Schrödinger the Cat purse. Shro' is a play off a Japanese legend that the souls of cats can come back as other things--in this case, a purse that a Mundane Japanese woman purchased of the Interdimensional Internet. Which meant hammering out the idea of Interdimensional trade (the details of which are still fuzzy, mind you), plus the whole InterDimNet, which brings up some fun in the next novel...
So, plot breeds character, character breeds world, world influences plot, while at the same time develops the characters.
Welcome to the inner workings of my mind!
BTW, I've posted the chat transcripts for last night's worldbuilding seminar and the launch party at www.fabianspace.com. Check out the news section on the homepage for the links.