Why this is a mistake: If the protagonist is exactly the same person
at the end of the book as she is at the beginning of the book, then the journey she underwent through the story served no purpose.
Yet many novice authors fail to give their protagonists adequate
character arcs. One thing to consider is that the protagonist, as she is at the beginning of the book, would fail to triumph in the final conflict. The growth and change she experiences, and/or the community she develops around herself as she goes through the story, are what ultimately allows her to defeat the antagonist when she reaches the climactic scene.
The solution: Three things signify growth or change for a character. The first is that the character must have a moment of enlightenment. This means that the character either experiences something she’s never experienced before, or experiences something familiar in an entirely new way. This is the lightbulb turning on. Characters—and people—have moments of enlightenment all the time (hopefully you are having many while reading this book). However, by themselves these moments mean nothing.
Second, the protagonist must make a decision based on the moment of enlightenment. In a novel, this decision often comes at the point of no return for the protagonist. Sometimes the decision isn’t necessarily a good one, as it leads directly to the moment of crisis, that darkest moment
when all looks lost for the protagonist. However, even this decision doesn’t mean the protagonist has changed.
The third and crucial signifier of change is sustained action.
This is what finally molds the protagonist into someone new. Then, in the climactic scene at the end of the story, the protagonist is able to defeat the antagonist because of all she has experienced.