6. Make the internal come external. Explore your protagonist’s internal needs and values, and consider, how will this affect her actions? The external events will cause internal change… and the internal change will cause new external events.
Example: In Silence of the Lambs by Thomas Harris, Claire (the FBI recruit) is shown in the opening as both charismatic and alienated from her classmates. She is popular enough, but sticks to herself. Even with her mentor/teacher, she is withdrawn and withholding. The question is posed, why is she so closed? This question becomes more focused when she is assigned to interview a serial murderer who used to be a psychiatrist. His canny reverse-interrogation reveals that her past (as an abandoned child traumatized by a gory event and "thrown away" due to her sensitivity) made her close herself off from others. But it's that very mix of alienation and empathy-- distance and intimacy-- that makes her a good profiler. While at first she's drawn into Lecter's seductive distractions, she can use her ability to distance herself to figure out what he's hiding and what she needs to know.
What's important here is that her "internal"-- the inner conflicts and needs-- draw her to this work and make her especially skilled at it, and that the "external"-- the plot events-- force the internal conflict to the surface where it can be revealed and perhaps resolved.... and then help resolve the external conflict (the mystery or quest or whatever).
So what is the protagonist's role in the external story/plot?
He might be the investigator, the one who must find the truth.
He might be the contestant, the one who wants to win.
She might be the leader of the team.
She might be the one on the run from danger.
She might be the helper.
She might be the healer.
He might be the one who subverts the organization from the inside.
He might be the one who invents the machine.
She might be the mother of the king.
I always try to ask, What does he/she do in the plot that no one else can do? If she's the mother of the king, she's the only one who can persuade him to lift the tyranny established by his father.
If he's the one subverting the organization, it's because only he has the cyber-skills to hack into the encrypted files, AND the motivation to bring down the company.
Then go inside-- how did she get to be the sort of person would would be in this role? What about him made him want to acquire these skills, or made him good at this? (For example, he was a frightened, secretive child, who had to learn hidden ways to deal with abusers and bullies.) That's the "internal".
Now think about what 'internal' motivates the protagonist to get involved and stay involved now, despite the obstacles and dangers? Like - she was married off young to the king, and gave up her freedom in exchange for wealth and luxury... and only now understands that she is in a gilded prison and has learned to sympathize with others trapped in their lives.
If you can identify the protagonist's role in the external plot, and also define the "why" of the internal motivation, you'll be deepening your story. The events will become the tunnel from the internal character to the external world.