When I am thickening my plots, I like to think 'What if ... What if ... ' Thus my imagination can move from the likely, which everyone can think of, to the unlikely-but-possible, my preferred plot.

 Patricia Highsmith 

The unconscious mind takes the germ of an idea and develops it, but usually this happens only when a writer has tried hard, and logically, to develop it himself. After he has given it up for a few hours, getting nowhere, a great advancement of the plot will pop into his head. I have been waked up in the night sometimes by a plot advancement or a solution of a problem that I had not even been dreaming about.

Patricia Highsmith