When people say "How do you write a book, how does it all happen?" I say, you line things up, and you line them up as actually as you possibly can, but sooner or later the book has got momentum and it's moving along under that momentum. It's like a sculpture, if you're working with the grain of the wood, the wood will start defining what shape it's going to become.
Generally I start writing when I have even the smallest idea of how a book is going to go, because the physical process of writing itself keeps the mind active and focused on the job at hand. Usually I write in about 5 drafts, but that simply means there are 5 definite times when I go in a linear fashion from the beginning to the end of the book.
It was Sci-Fi and fantasy that got me reading, and Sci-Fi writers in particular have pack rat minds. They introduce all sorts of interesting themes and ideas into their books, and so for me it was a short leap to go from the fantasy and Sci-Fi genres to folklore, mythology, ancient history and philosophy. I did not read philosophy because I set out to become a philosopher; I read it because it looked interesting.
There's that lovely thing for the first month or two of writing a new book: OK, I don't know what that character's going to do, but we'll find out later. After about three or four months you come to that bit where you've got to put some plot in before it's too late, and you have to go back and start inserting plot, and, ooh, I've left out the literature, OK, lets put some in.
If you get the characters right you've done sometimes nearly half the work. I sometimes find I get the characters right then the characters will often help me write the book - not what they look like that's not very important - what people look like is not about their character. You have to describe the shape they leave in the world, how they react to things, what effect they have on people and you do that by telling their story.
Aziraphale collected books. If he were totally honest with himself he would have to have admitted that his bookshop was simply somewhere to store them. He was not unusual in this. In order to maintain his cover as a typical second-hand book seller, he used every means short of actual physical violence to prevent customers from making a purchase. Unpleasant damp smells, glowering looks, erratic opening hours - he was incredibly good at it.