Characters -- not just a compilation of physical and personal traits. The difficulty a writer faces when developing the characters for his/her project is complex. I've heard comments from those who say it took pages and pages into writing the story before really feeling like "knowing" the people in it. Can you imagine if the reader felt that way? If I had that much difficulty figuring out what the character in a book is about, why she is doing the things she's doing, I don't think I'd have the patience or the concern to keep on reading. I just wouldn't care.
So, with that said, it should seem to you very important to find out everything you can about your characters before you cut them loose on their journey through the plot of your story. The question is how? What should you do, what steps should you take to get to that point? First of all, you need to remember that story characters are people like you and me. Only they are in your book. To make them human means there are several points to consider. The character's motive, habits, interests, talents, past history, reputation are all important to development. Identifying these will turn your story people into real people who readers can identify with and sympathize with. They will become so invested in the characters that reading on until the very end becomes a must.
The tiny details are important. You can keep molding and adding those aspects to your characters, fleshing them out until the decisions and actions they perform in the story seem logical and believable. For instance, let's consider habits. Perhaps you could give a character the habit of chewing his nails to show a nervous personality, or someone who always doodles on her napkin after a meal, and then later a napkin is found at a crime scene. It has scribbles all over it, thus providing a clue. The character's interest in judo and the fact that the murder victim taught a judo class at a gym your napkin scribbler frequented makes the reader point a finger in her direction. These are just a few examples of how intricate the process of developing believable characters can be.
Where you get your ideas may vary. Strangers you observe, yourself. And sometimes characters are inspired by people you know. This works if you use them only as a starting point. From there, you should develop them according to what you'll need for your story. Flesh them out with those tiny details of habits, talent, motive, interests, etc. Then ask questions based on your story events. For instance, you want to write an opening scene where the character is home alone. There's a pounding on the door, someone shouting, demanding to be let in. Now, start the causal question process: what does the character do? Remember this depends on all those aspects you've created about the character. If it's the nail biter, maybe he'll hide in the closet, pretend he's not home. Next question: The guy breaks down the door and finds the nail biter. What does he do? He has a brave moment and uses the baseball bat stored in the closet and hits the intruder. You could insert a plot twist here: turns out it's nail biter's brother who has come to tell him his wife has been in a serious car accident. And the question process takes a turn and goes on from there. The point is, the better you know your characters, the easier it is to decide what to make them do, how to act. And the more believable they become to the reader.
For more about character development, a great source is Characters and Viewpoint by Orson Card.
