You have spent a lot of time developing your characters. You’ve put together their back story and taken the time to really understand how this has impacted who they are today. You’ve drawn up a lot of stories and scenarios that won’t make it into your writing but have helped you get a better understanding of who they are. How can you help them evolve throughout your story?
Your characters are the way they are for a reason, and this has somehow created conflict in their lives. Something that happened to them, something they did, or something that was done to them created an issue that must be solved, and now you have a story to write. Your characters are going to initially look at the situation from one angle, and seeing that it isn’t just a simple fix, will need to work hard to determine what they can do to create a satisfactory outcome.
This is where their development will take place. They may have a chance to grow from shy kids into powerful adolescents who aren’t afraid to speak up for themselves or decide that the friend they’ve been holding onto for years is better off left in their past. They may figure out that they were lied to and uncover disturbing truths or find that they in fact caused the problem they have been blaming on others. Whatever the case, they will need to change what they’ve been doing if they are to solve the problem.
Think about what is realistic for your characters and try to determine what makes sense for them as far as changes are concerned. Take their personality into account when doing so and make sure you create believable scenarios. For example, someone who is shy and gullible isn’t going to suddenly stand their ground with an intimidating individual. They will need to develop the ability to do this and the situation must be dire enough for them to be willing to make a stand. Conversely, an extraordinarily stubborn individual isn’t going to just change his mind about something. He may spend a while heading in the wrong directions because his personality makes him persevere more than most. He may also need a lot more convincing when it comes to changing his mind than another character might and this personality may cause conflict between them.
Your readers want to see the story develop along with your characters so you will need to make a point to meet them at their starting point and bring them along for the ride. Take the time and effort to evaluate your characters reactions and decisions along the way to make sure they are in line with who your characters are and what they would naturally choose given the circumstances.