A plot, after all, should never be a rigid thing in the writer’s mind when he starts to work. I carry this thought one step further and believe that a plot should not even be completed. I have to think of my own entertainment, and I like surprises myself. If I know everything that is going to happen, it is not so much fun writing it. But more important is the fact that a flexible plot line lets the characters move and make decisions like living people, gives them a chance to debate with themselves, make choices, take them back, make others, as people do in real life. Rigid plots, even if perfect, may result in a cast of automatons.
From Patricia Highsmith, Plotting and Writing Suspense Fiction (1983).