How can I make a cautious scholar protagonist develop into a decisive leader?

[William54]

I'm writing a fantasy novel and my protagonist feels frustratingly flat—she has a clear goal and backstory, but her reactions and decisions feel predictable and lack depth. I want her to have a compelling arc where she evolves from a cautious scholar to a decisive leader, but the transition feels abrupt in my drafts. For other fiction writers, what techniques or exercises do you use to develop nuanced characters whose growth feels earned and authentic over the course of a story?