… one day quite suddenly I knew that I was ready (to find and reconstruct Arthur), that the time had come when I really could cope with it. Most of the actual research I did for the book, apart from knowing the Arthurian story from the romance versions, was into Dark Age life and history as far as they were known. Then I worked into this setting the Arthur who seemed to me to carry weight, to be the most likely kind of person. It was very strange because I have never written a book which was so possessive. It was extraordinary–almost frightening.
It took me about eighteen months to write, and it absolutely rode me throughout the entire time. I would take the book to bed with me at night, and work there until I dropped off to sleep about two o’clock in the morning, too tired to see any more. Then I would wake up about six o’clock, still thinking about it. It was addictive. It was almost like having the story fed through to me, at times. I do my writing usually in three drafts, and I would go from the first to the second draft, from the second to the third, and find bits of the book that I had no recollection of having written at all. It was interesting, almost scary, but much of the material had this effect of being almost fed through to me, rather than being the result of my own research.
Source: University of Rochester