Our plot ideas come from a multitude of sources and are sparked by many things. Oftentimes, the germ of the idea that sets our plot into motion is forgotten as the idea grows. But whatever gets your plot idea moving, always remember, you characters make your story. Good characterization can even hold together a weak plot.
You should know your characters as well as you do your spouse, your children, your siblings--maybe even better. You may never get to tell the reader everything you know about your characters, but the more you know, the more successful you'll be at creating well-rounded people. And in the course of the writing, many of the small details that you know about a character will be revealed through dialogue, action, and interaction. They might even become major factors in the plot.
Working out an outline will enable you to do this, and a good place to start is by drawing up a character sketch of each person. Ask yourself everything you possibly can about a character. Question yourself about his childhood, family, past, previous romances, experiences, desires, goals, likes, and dislikes. Determine what he would do in certain situations.
Unless the character is schizophrenic, his/her behavior and basic motivation should be consistent throughout the book. People in real life aren't always like this, but generally speaking, you can take someone you know very well and determine exactly what his/her reaction will be to any given situation. You should be able to do this with your characters, too.*
Next week: MOTIVATION
COPYRIGHTED MATERIAL: *From my 1992 RWR article: "The Outline: Your Blueprint for a Structurally Sound Plot."