I don’t remember which it is. But whatever it is, it’s something you do to your characters before (or maybe while) you draft your book. Briefly, let’s look at a couple ways to make your characters flesh and blood — breathing and thinking and living human beings.
1. Know his story: So during your novel, your character solves a bank robbery and falls for the bank robber. But what happens before the story? Don’t just go for the general, like the name of their hometown and highschool sweetheart. Know detailed scenes. Know their thoughts. Know their lives. Sophie, the main character of THE INBETWEEN, doesn’t just show up in Morrow, Oregon and plunge right into a new life. She carries a story: a life defined by playing the violin and performing with her brother. I don’t use all the details I know about her life in the book, but they still add depth to her character.
2. See the world through her eyes: Spend some time walking around town, going through the grocery store, or just doing daily things while thinking of your character. How would he or she describe your day to his/her friend? What details would they highlight? Leave out? Laugh about? What kinds of similes and metaphors would they use? In one of my earlier novels, Patricia (the main character) was a cook. She saw everything through a cooking lens, so she used food imagery. Sophie plays the violin, so she sees and hears music everywhere.
3. (This one might be weird to some of you. So prepare yourself. I promise I’m not crazy; a thousand other writers will vouch for me) … Talk to them: Yep. For the longest time I thought people who had ‘conversations’ with their characters were kind of bonkers, at least until I got to that level myself. It took about two years of writing, but I finally reached the spot where I knew my characters so well that they really, really felt real to me, as real as if they might walk into my room and say hi. Then they started saying things. Arguing. Short example: Patricia the cook was supposed to like her friend Ben and get together with him by the end of the book. But her other friend Reed would not settle for sidekick-status. He was so insistent that I did major surgery on the book and let him have his way. It’s actually a better draft. So listen to your characters. They often know what they’re talking about.
Mostly, although you do want to know your characters inside and out, don’t be afraid of being surprised. Characters might not do what you want them to and might not become the people you want them to become, but that’s okay. It’s kind of exciting, actually, because someone like Reed (whom I had characterized as slightly-nerdy-sidekick) will completely blow your mind by turning out to be the slightly-nerdy-boyfriend instead. Cool.
I also use this character chart sometimes when I’m first fleshing/flushing out (I think it’s fleshing, actually) a character. It’s a good tool, but don’t let it become a prison. Let your characters stretch its boundaries as you write.