When I first started writing, I had little to no idea on how to craft a character arc, or what was involved with plot development and appropriate process. I simply wrote what I felt, what I saw in my mind, and hoped for the best.
My former agent taught me about GMCs (Goal, Motivation, Conflict) and how they apply to storytelling. These aspects establish your characters progress through your story, and given depth to their actions.
This technique provides your character with a Goal (what they want), Motivation (why they want it) and Conflict (what is holding them back from achieving their goal), with both internal and external issues. Consider this early on, and it will help you develop your characters arc as they move through the story.
This is what your character wants. It can be something as simple as getting in to their dream college, or to survive a life or death situation. This is what drives everything they do through your story, so make it something significant, and that they hold to throughout. Readers may share similar goals to your MC, depending on the type of story you write, so consider this as you develop their goal.
External goals are things that are outside of the character themselves. They are external to the character, with external effects to reaching that goal.
Internal goals are just that; internal. Emotional, cognitive, feeling. They can still be attached to another person, such as forgiving someone for wronging them, but much of the goal is felt within the character themselves.
Example: In the Wizard of Oz, Dorothy wanted to get back to Kansas. That was her goal. To go home.
When writing about this particular aspect, try to be as in depth as possible. Why do they want what they want? Why that school? Why do they want to solve that crime? This creates another level to your character and who they are, by describing why they feel as they do.
There are times when their motivation isn't necessarily their own, either. Take Wattpad hit The QB Badboy and Me by Tay Marley. Drayton's goal to go to college in Dallas was not his own. It was his fathers, and yet, it was a still a goal with its own motivation. The conflict, was that he didn't actually want it for himself. When you aren't sure how to apply these steps, look at some of your favorite stories and break them down. It can help clear the fog.
This is a step that you can have some fun with. What is stopping your characters from reaching their goals? Is it self doubt? Is it a rival? These conflicts can create additional levels to your story depending on their reason and place in the tale.
Again, these are internal and external, just as with the goal and the motivation. Using the chart at the top of this chapter can help you keep yourself organized.
These considerations do not just belong to your MC, either. Your antagonist plays a role in your MC reaching those goals, and they have their reasons for it. They, too, have their own specific GMCs that work within the storyline, so during this planning phase, be sure to fill in the GMC chart for all significant characters.
If your story is a multi POV, such as One of Us is Lying by Karen McManus, the GMCs need to be completed for all characters, as each have their own path in the tale, own goals and motivations.
This is one of the most significant tools in writing a well rounded story, and developing your characters arc through the pages. Whether you are a plotter or a pantser, this is one step you shouldn't skip.