Pre-Production - Plot Detailing - What is the Change?

By this blog post, I mean the change in characters. In any great story or drama, there is a change. For example, Harry Brown. Harry goes from a simple old man with an army past to a ruthless killing machine, because someone butchered his friend. In Taken, Liam Neeson was a security guy with a past in secret service, but it takes somebody to kidnap his daughter for him to trigger. In my story, I want the change to be the change of behavior in Jimmy, and Billy's attitude from happy go lucky to somewhat of a detective to find out what Jimmy is doing whilst he is not studying.