I tried to find the original thread, but after 13 pages, decided to restart the thread from scratch.
Writing is an exploration. You start from nothing, and learn as you go.
~ E. L. Doctorow
This is particularly apt, as it not only applies to learning how to write, but also details the process of writing any particular book: watever your plans, you essentially start from nothing, and learn about your characters 'as you go'.