Make the character "round." This means the character must be three-dimensional. Many works of fiction use two-dimensional or "flat" characters; these characters are like walk-ons in a movie, play or TV show. They appear briefly, are stereotypical heroes or villains or any other stereotype, and serve a specific purpose, such as interacting with a main character. However, if all the characters are "flat," it will be difficult for readers to get interested in your story. An in-depth, or "round," character must be much more well-developed than these. He or she must have a unique personality, physical appearance and background, as well as thoughts and feelings.
Give the character qualities that make him or her lifelike and realistic. In-depth characters have a past, a personality, thoughts, feelings and motivations. They have their own specific manners of speaking. Some even have accents. Incorporate as many of these elements as possible to give your character more depth.
Make the character behave in logically motivated ways. In serious literature, the motivation of the characters is important. When creating your character's actions, make sure that they are things that make sense for the character to do. For example, it does not make sense for a character who is afraid of heights to wash windows on a skyscraper. Whatever you have your characters do, make sure there is a reason for it and be sure to make the reasons clear to the reader.
Make your characters behave as believably as possible. Once you have given your character a certain unique personality, have him or her operate within the parameters of it. For example, if your character is a shy and introverted outcast in high school, it would not make sense to have him or her run for class president. It would make sense for her to have a crush on a boy and never tell him.