How to write a novel..

...with the most efficiency?

I guess there are many different ways to write a novel. This is only my third that I'm currently working on, so I don't know whether I've hit on the best method for me, or not; but I have noticed a pattern emerging.

What I do is, work out what the main characters are like, in quite a bit of detail; I know their childhood pets, first boyfriend/girlfriend, if they drink tea or coffee for breakfast, which bits of their appearance they dislike etc., and also what the book is about; the themes and outline of the plot. I make pages of notes on the characters, and jot down random ideas. I know the end of the novel, and write the key scene that occurs at or near the end.

Then I start writing. I usually know two or three scenes that I will write, and by the time I've written them I've thought up the next few. Like a walk through a dark wood with a torch. I make up other characters as I go along, and let them play off against each other. It's scary, because how do I know I won't run out of ideas?

I've tried writing scene titles on cards, doing time lines, and attempting to map out the whole structure of the book before I begin, but for me these methods didn't work. If I have a scene in my head, I need to write it down quickly before it fades, not put it to one side because my outline isn't finished yet.

How do you do it?