The introduction of the concept of character change should occur right at the beginning of the story and continue throughout the novel. The best way I can illustrate this process is with a simple scenario.
At the opening of your mystery novel introduce the main character in her natural environment. Let’s say she’s a schoolteacher, settled into and contented with her day-to-day activities. She minds her own business and stays out of trouble. She figures she’ll continue as she is forever.
Early in the plot, an external event triggers a low-level internal conflict. One day our protagonist reads a newspaper ad soliciting participants for a study abroad summer program in France. Should she fill out the application? Maybe. Probably nothing will come of it anyway. She starts feeling discontented with her mundane life. She ponders changes.
She applies, is accepted into the program, and flies off to France and a radically different lifestyle. She returns to the U.S. and takes a job teaching French in a community college. A small change has occurred, the foundation for further changes.
Now other obstacles arise in the form of an antagonist. Her college colleague is murdered. Things she’s witnessed lead her to suspect the department head who hired and supported her. Internal conflicts plague her: Should she alert the police to her suspicions? The department head has helped her in her new career. Why would she betray him? Does he have an idea she suspects him? What if he’s now desperate and dangerous? She fears for her own life. She feels like an ingrate and a coward. She longs for her previous dull but safe life. She longs to return to her old ways of dealing with obstacles. Avoid them. Ignore them. Step around them.
She discovers her old way of doing things has stopped working for her. She decides the death of her colleague is her business. She can’t ignore it. She needs more facts before she goes to the police so she starts investigating even though she finds herself in several dangerous situations. She doggedly pursues her goal until she discovers the evidence the police need to convict her department head. Our protagonist has now embraced her new way of doing things. She has overcome all the internal and external conflicts she’s encountered. Her values, habits, and attitudes have changed over the course of your mystery story.
Conflict produces change in a mystery novel’s main character.
More writing tips: