“Cardboard characters” are those that have no personality. Make sure yours have likes, dislikes, and opinions so they act like real people. If necessary, keep a list of each character’s physical and personality traits on file if they’re not vivid enough in your mind without it. Readers notice if your hero’s eyes are blue on one page and green on another. Same goes for that couch or car! Never underestimate how astute your readers may be! Otherwise you’re likely to find out when they give you a lousy review.