In developing the characters for P2PC, I realized that the three principle characters all have something in common; they are refugees from their pasts--each in a very different way. Each suffers from grief and confusion over a life-altering event. They have all endured personal torment. They all have inner demons to defeat. One of the characters even seems to have benefitted, until the reader is clued in how their life was impacted. (Yes, I'm intentionally being non-gender specific here.)
P2PC has three protagonists--one male and two females--though two of the protags become enemies. In a twist, the lives of the two who are at odds end up having close parallels. It takes a violent confrontation to reveal the common ground.
The characters all deal with their loss differently--by escaping from it, by swearing to avenge it, and by living in denial. Eventually, they all have to face it.
Backstory is always one of elements I enjoy discovering, whether I'm writing the tale or reading it. The novel begins at a place and a point in time, but what came before defines how the characters will act, feel, and respond to other characters and their environment. It's the buried treasure chest waiting to be uncovered. But what's inside? Is it bright and shiny or dark and twisty? (Dark and twisty is so much more fun.)
When I sit down with a story idea and begin working, I don't know every nuance of the characters or their pasts. These elements come to light as the story evolves.
I think that's why writing is such an addiction for some (*raises hand*), because it's constant discovery and you never know what surprises and revelations await you. Writing a story is, as Forest Gump said: "...like a box of chocolates. You never know what you're going to get."