We do it through character bibles.
Character bibles are the homework we do before we start writing our manuscript. They are imperative in my opinion! In our character bibles we write out, in narrative form, the most impactful moments of our character’s lives. Everyone on this earth who has lived to the age of 1 has already had a series of successes and failures. Failures scar us and victories provide that euphoric feeling that we spend the rest of our lives chasing. You have to intimately know the fears and desires that battle within your characters every single day of their existence, if you expect to create characters who will surprise you throughout the writing process and surprise your readers as they turn the pages.
Consider this: I have a female character with a fear of being abandoned (based on a father who left her and her mother at a young age and was never heard from again). She also has a desire for wealth (to support her dream of opening her own clothing boutique). What will she do when she is propositioned by a wealthy gentleman who is crazy about her, but has a history of running off on the next great adventure? Her desire for wealth and her dream of opening a clothing store battle her fear of abandonment. Which will win out is based on the depth to which she feels the details of those fears and desires.
So in our character bible we explore the specificity of those fears and those desires. We analyze the moments of her life that relate to her father abandoning her when she was young. We write out three prominent memories of what it was like to not have a dad in their most painful reincarnations. The day she went to a friend’s birthday party who was turning five and her father had built her a dollhouse from scratch. The day her own mother told her that she couldn’t go to camp that summer because mom was holding down two jobs and still couldn’t afford to send her. The day that she finally felt the loneliness her mother went through and endured just trying to keep a roof over their heads.
Each of these moments is profound. So we write a narrative three pages about each event. We explore what it looked like, felt like, smelled like. What stood out about the moments, about the players, about the setting. If we let ourselves go to these places, we begin to feel sympathy for our characters and that means the process is working. We all have these scars. Everyone’s is uniquely theirs. Conversely we all have tremendous victories. Explore those with as much detail.
Once you have taken the time to develop the peaks and valleys of a character’s life through narrative, descriptive writing, your characters will start to take control of their lives. It is that point that they are ready to start playing with other characters. Like toddlers fumbling around, sometimes running, sometimes falling. This is what you’re after. This will define their motivations.