My process

Novels come to me as short movie clips, only a few seconds. In When the Lantern Swings, I saw a cop standing over the mangled remains of a young woman. It was obvious to anyone that she had been run over by a train, but that was impossible because the tracks had been taken out forty years ago. Then I go about building the story around the clip. For each of the main characters, I build a complete backstory, even if I don't use it all in the novel it helps to know what they've been through and how it informs their actions.