Whether you are looking for a way to better define your player character’s backstory, thinking about creating a new NPC or playing a mission related character one critical thing to think about it what prompted this character to take action?
In some role-playing systems this is called a “kicker” which is short for the thing that kicks your character into action. The kicker is an event or realization a character has just before a story begins. This functions as a catalyst that propel characters into taking action, whether that is action for the cause of the mission to to antagonize the mission.
You can think of this like the opening scene of many movies. Before the kicker, the focal character whether they are a protagonist or an antagonist is just some person. Maybe a good person, maybe a bad person, but one that is living a static life. The kicker pushes a character forward into action and that action becomes a story.