I received a grant from The Ford Foundation to write a book for kids about urban perception, or how people experience cities, but I kept putting off writing it. Instead I started to write what became The Phantom Tollbooth
And when I'm writing, I write a lot anyway. I might write pages and pages of conversation between characters that don't necessarily end up in the book, or in the story I'm working on, because they're simply my way of getting to know the characters
I write best in the morning, and I can only write for about half a day, that's about it.
The only other thing which I think is important is: Don't write a book or start a book with the expectation of communicating a message in a very important way.