Typically, plots of my novels start with a collection of images, moments, vignettes, and other idea snapshots that feel related to me, although I do not always know why. Gradually, I discover the connections as the planning and writing evolve. During that evolution, there will always be ideas that turn out not to fit, after all, and I have to scrap those.
Similarly, my characters start as a pastiche of attitudes, actions, and problems, which may be drawn from people I know, situations I have experienced, or stuff I’ve overheard in passing. (Beware discussing your life while standing in a grocery store line. There may be an eavesdropping writer nearby.) As the book progresses, I inevitably discover that multiple characters have conflicting traits that all belong to me. Real humans tend to be more contradictory than even the most complex of characters. Perhaps on certain levels I use the characters to work through some of my contradictions.