I plotted each character's prior history, and imagined their personality quirks, even if I don't have room them all in the story. Through doing this, I came to love even the antagonists. Knowing the reasons for their behaviors, it was hard to allow Dora to be such a poor mother, to let Barry fail so often, and allow Ceci to suffer.
It also pays to learn the backstory in real life. A mere mile's walk in someone else's shoes would never reveal the alcoholic parents, the kindnesses done in secret, and the dreams yet unfulfilled. We respond differently to others when we hear their stories.
I'd love to know your favorite characters in the book. What do you like about them? If I were to write a sequel, which character would you have me feature?