Get a bunch of 5 by 7 cards and put each character’s name at the top. Next, think about the role each plays in your story, and what kind of person each is: age, education, place of birth, hot-headed, funny, fat, ugly. What are their quirks? Do they wash their hands 500 times a day? Do they hear voices? Are they kind to kids but love to torture cats? Put it down, put down so much that you finally come to know these characters intimately. Alfred Hitchcock would write down his scenes on index cards, one scene to a card. That way, as he said, by the time he was ready to shoot the film, he was already done.
     Some characters will be major ones, around whom the story will pivot; others will play bit parts, but these will be critical too, as every player must have a reason for being in the story. If they don’t have a reason for being in your novel, they’ll slow down the story, and slowness bores readers.