Writing a book is like pregnancy: once the idea is conceived in your brain it gets a life of its own.
It grows at its own will giving you morning sickness because you have to go to work while you'd rather stay at your computer keyboard writing.
During the day you'll experience mood swings: anger at anyone or anything that's in your way delaying your writing, excitement when you manage to think of an interesting twist of the plot. S
uddenly there are those people in your head who do things, say things and you have to rush and capture it all before it vanishes into thin air buried under the trivia of the day.
They are with you all day long, you can feel them move in the most unexpected, sometimes awkward moments (like when you are having dinner with your family or a professional conversation and a character stirrs in your mind causing you loose track of your thoughts and concentrate on your inside instead).
You start trying out names on your tongue to see whether they would fit the person being created in your head.
You don't really have control over it all, it just unwinds before your eyes until the story is ready.
Then you let it out into the world with the tentative hand of the parent watching over it as it takes flight, thinking: 'Will it succeed?' But you have to let it go like a grown up child and live its life, for keeping it with you would be just selfish.
The first seed of a novel feels like a miracle, that early stage when the murky idea takes shape and characters slowly become real in my mind. Then the enormous struggles-- both physical and mental-- and wonder if the book will ever really happen. Then, about nine months to a year later, the book is 'born' and share it with the world.
Like a child, a story starts to have a life of its own even before it gets published. Until you experience it, it is impossible to understand what the writers are talking about when they say how they let the characters write their books and can never be sure how the story will end, since it writes itself.