From The End of the Affair, by Graham Greene:
I was trying to write a book that simply would not come. I did my daily five hundred words, but the characters never began to live. So much in writing depends on the superficiality of one’s days. One may be preoccupied with shopping and income-tax returns and chance conversations, but the stream of the unconscious continues to flow undisturbed, solving problems, planning ahead; one sits down sterile and dispirited at the desk, and suddenly the words come, the situations that seemed blocked in a hopeless impasse move forward; the work has been done while one slept or shopped or talked with friends.
With the novel I’m writing now, I feel like I’m well into the stage where the characters have begun to live, and of course there’s a lot of pleasure in this. But I still struggle with the fear that grips me whenever I sit down to write, the fear that I won’t be able to move forward. So it’s always nice to remember–or at least to hope–that the subconscious is always at work, and that progress may be right around the corner.