You'll find a variety of check sheets and questions to ask yourself online. These can help you think of traits to keep track of, but initially, you'll want to write down the name, where they live, age, occupation, and any other pertinent information. Then start thinking in terms of personal traits and characteristics. What do they look like? What goals do they have? Do they dislike children, afraid of snakes - anything else that defines this person. The more ideas you write down, the better idea you have of who this character is in terms of the story. You may not use all the things you write down, so start with the most important and filter down.
When you are writing a scene with this character, open your notebook to that page. Six chapters after you mentioned the name of the place where they work, you may not remember. Your character sheet will remind you of these basics and save you lots of time scrolling through your story.
One way to make this person interesting is to give them a trait that sounds completely off the wall and in contrast to everything else. You may have a kindergarten teacher who loves bungee jumping or a preacher who dances the Rumba in his living room for exercise. Explaining how these things came about can make for an interesting story and dimensional characters.
If you have already started your book and haven't done character sheets, do them anyway and watch your characters come alive.
