I've been corresponding with a fan of Bitterwood who's asked me for some advice on developing the characters in a story he's working on. I told him that, for me, one of the keys to figuring out who your major characters are is to figure out what the worst moments of their life have been, and know what lessons they took from these experiences.
One reason I start Bitterwood out twenty years before the main novel is that I wanted to tell my readers about a pivotal moment in the hero's life. The story starts with Bitterwood's brother on the verge of raping the woman Bitterwood loves. Bitterwood is powerless to stop his brother Jomath, but the rape never occurs because of the arrival of the Bible-thumping, axe-wielding Old Testament style prophet Hezekiah. Hezekiah proceeds to kill Jomath for being an unrepentant heathen; he kills a lot of other people too. Bitterwood is standing there witnessing this; his brother is dying at the hands of the Lord's messenger just at the moment when Bitterwood hates Jomath the most. Bitterwood comes away from this event with a clear vision in his mind that God is a god of wrath and vengeance. There is a higher force in the world watching out for Bitterwood and smiting those who have wronged him. Only, as the backstory unfolds, Bitterwood loses his faith in God when he finds out that Hezekiah is a false prophet. Then, Bitterwood becomes his own absent God; since there is no higher power dispensing punishment upon the wicked, he will do the job himself. He's not doing it for love or money; he's doing it because he wants the wicked to suffer, because that's the god shaped hole in his world that he needs to fill.
Of course, any rational reader will conclude that this is a fairly insane lesson to draw from the events of his life. Bitterwood would have been much better off, probably, just sucking up the injustices visited upon him and trying to rebuild his life. But, this is the second key aspect to character building: Characters are more interesting if they have learned the wrong lessons from life's traumas.
Take Bruce Wayne. He saw his parents gunned down when he was a child and drew the lesson that he should dress up like a bat and use his vast wealth to buy bat-shaped cars and airplanes, so that he could hang out in alleys and throw boomerangs at muggers. An alternative lesson he might have taken away would have been to use his vast wealth to try to strengthen police work and work for social justice to reduce the forces that give rise to crime in the first place. But, since he's learned the wrong lesson, he's interesting. If he'd learned the right lesson, who'd care? There are surprisingly few stories that feature well-adjusted philanthropists as their protagonists.
So my two sentence guide to writing interesting characers is this: 1. Know the event that sent the character's life off the rails. 2. Understand the major flaw in their world view that results from this event.
Obviously, this is oversimplified. But, if you're a writer trying to come to grips with who your character is, try the two sentence guide. Let me know if you like the results.