Martin Amis describes how he starts a novel and, in many ways, this gels with me too.
“The common conception of how novels get written seems to me to be an exact description of writer’s block. In the common view, the writer is at this stage so desperate that he’s sitting around with a list of characters, a list of themes, and a framework for his plot, and ostensibly trying to mesh the three elements. In fact, it’s never like that. What happens is what Nabokov described as a throb. A throb or a glimmer, an act of recognition on the writer’s part. At this stage the writer thinks, Here is something I can write a novel about. In the absence of that recognition I don’t know what one would do. It may be that nothing about this idea—or glimmer, or throb—appeals to you other than the fact that it’s your destiny, that it’s your next book. You may even be secretly appalled or awed or turned off by the idea, but it goes beyond that. You’re just reassured that there is another novel for you to write. The idea can be incredibly thin—a situation, a character in a certain place at a certain time. With Money, for example, I had an idea of a big fat guy in New York, trying to make a film. That was all. Sometimes a novel can come pretty consecutively and it’s rather like a journey in that you get going and the plot, such as it is, unfolds and you follow your nose. You have to decide between identical-seeming dirt roads, both of which look completely hopeless, but you nevertheless have to choose which one to follow.” [from The Paris Review]