I don't do all the background and the worldbuilding before I start the story. What I do is I work out the bare minimum I need to start the story, and often that really is a bare minimum - it's a character in a situation, and I know nothing about the character, I know nothing about the situation, and then I think about it for a long time, and make notes about where I think the story is going to go and so on, but I don't really make notes to do with the background or the magic system or the world.
Garth NixA Kiss," said Mogget sleepily. "Actually, just a breath would do. But you have to start kissing someone sometime, I suppose." "A breath?" she asked. She didn't want to kiss just any wooden man. He looked nice enough, but he might not be like his looks. A kiss seemed too forward.
Garth NixKnowledge, like all things, is best in moderation," intoned the Will. "Knowing everything means you don't need to think, and that is very dangerous.
Garth Nix