I always have a basic plot outline, but I like to leave some things to be decided while I write.
I’m a big fan of outlining. Here’s the theory: If I outline, then I can see the mistakes I’m liable to make. They come out more clearly in the outline than they do in the pages.
By about chapter six of ‘Wolf Brother,’ I was having so much fun that I knew I wanted it to go on and I couldn’t tell Torak’s story in one book. So I sat down, and it took me about a week to plan in broad outline all six books.