The Characters in your novel have layers, just like ogres, and onions, and parfaits. If for some reason your want your Characters to be flat and boring, then read Rudy Anaya. But if you want your Characters to be interesting and stimulating, each needs to be as unique and individualistic as possible. There are a few things the Creative Writer may want to keep in mind to spread out the dimensions of your Characters.
We are all shape by our childhood, so why wouldn’t our Characters be, as well? You don’t have to detail the events of childhood to show how they affect the Character. For example, James Tyrone, the patriarche of the Tyrone family in the Eugene O’Neill play Long Day’s Journey Into Night, is a skinflintery miser. That may sound like it feeds into every Irish stereotype, but O’Neill lets it slip that James grew up dirt poor, so he felt compelled to cling to every cent he could. This not only shapes him as a Character, but feeds into the story itself, in that James is too thrifty (let’s be kind) to spend money on a good doctor for his wife or his youngest son. Even as a much older man, his actions, based upon the type of person he really is, are shaped by his childhood.
There are some novels where the relationships of all the Characters is the story. But even in tales not so based, the Characters in all tales are affected by how they relate to others, friends and family, and foes alike. Pick one of your Characters. Did he get along with his parents, or does he still have daddy issues? Is he married? Does he dote on his wife, or is the marriage strained? Who are his friends? How does he treat them? It is clear how these relationship go far in determining what kind of person each of your Characters are.
What your Characters do for a living matters. Moby-Dick just wouldn’t work if Ishmael sat on the Executive Board of Greenpeace. In Faulkner’s The Hamlet, Flem Snopes is who he is based upon his work, but only because it demonstrates his takeover of the town – from the general store to the horse exchange, the story reveals him basically because of his work as it reflects the advance of his Character.
It is easy to give provide pastimes for your Characters that mirror your own interests. I would love to for all my heroes to be baseball fans. Certainly, I’d write about what I know, but there needs to be variety in my Characters, so there needs to be a difference in their hobbies. Maybe I need some Characters who enjoy opera and others who listens the Opry. While I’d like for my favorite Characters to de dedicated to Mozart, maybe I should have someone listening to Cool Moe Dee once in a while. But the hobbies should reflect the personality. A rock shaper should have a patient and meticulous nature, while a photographer might by known for his perspective on the world.
We all have an outlook on life. Our Main Character may be Catholic or an Atheist, Republican or Democrat, given to hedonism or dedicated to sacrifice. Worldview has everything to do with what a person wants, and what someone wants moulds the individual motivation. Our Characters need motivation, each must want something very specific. What motivates out Characters begins with every one of their worldviews.
All these add layers to your Characters and makes sure they stay interesting. But this is of course only a starting place. You can think of many other layers to add to the richness of their personalities and details to fill out their being.