Let's say you've come up with some characters. Heroine, hero, villain. Protagonist, antagonist. Sexy vampire sidekick. Whatever. You give them names, hopefully ones that are pronounceable. Hair color, eye color, height, body type, all that kind of stuff. Or maybe you pick an actor to serve as the model for your character's physical description. (If anyone knows of any smokin' hot alpha werewolves modeled after Tahmoh Penikett please send me the Kindle buy link.*) Next you start working on their personality and all the little details that make up who they are. Maybe you give them a birth date and consult astrology charts. Maybe you do a tarot spread. Maybe you just throw a bunch of random stuff together. "Likes Mexican food, Miles Davis, and manga." The character's history includes a bad childhood, a spouse murdered by a serial killer, or even a charmed life until Page One. Whatever, it doesn't matter. Well okay, those these do matter, but none of them mean as much to your story as one thing:
What does your character want and why do they want it?
Last week I talked about conflict; motivation ties in directly with conflict. They are some of the most essential building blocks to a compelling story. Your character wants something, otherwise there's not much point. But why do they want it? Their reasons for wanting to reach their goal will tell you how far they are willing to go to get it. Motivation raises the stakes, which supplies conflict. You've got to know why your character is so invested in their goal. If your portrayal of a character's motivation is too nebulous and unformed, your reader will find it unconvincing and will soon cease to care. You may not need to share every single detail of a character's back story in the narrative, but writers need to know it all even if readers don't. Especially anything related to their motivation.
The answer to the question why is at the heart of any story. Find the why, the motivation, and a huge chunk of your story will fall into place.
*I was just going to link to a pic of the actor but figured I might as well post it. Battlestar Galactica is the latest thing I've been watching on Netflix Instant. Ah, Helo...