For all my longer works (i.e. the novels) I write chapter outlines so I can have the pleasure of departing from them later on.
Garth NixIโm not going,โ he said. He held up the Third key like a weapon. Sensing his mood, it grew longer and sharper, till he was holding a trident as long as he was tall... โAnd anyone who tries to make me is going to suffer.โ โTwice,โ added the voice under the table.
Garth NixI 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 Nix