I'll admit it. As a child, I saw monsters in the bathroom and heard wild animals outside my bedroom window. I loved watching Scooby-Doo on Saturday mornings and as I laid in bed on Saturday night, I was sure that a goblin or witch was hiding in my closet or under the bed. Some might say I had an overactive imagination. Others might think I still do.
Imagination is the portal that carries us to other places - whether an imaginary realm, a fictional city, or a table at your favorite cafe. It introduces us to characters - knights, fairies, detectives, and the girl next door.
As writers, we want to tell stories that our readers will want to believe. Just as the laws of nature are constant in the real world, we must set rules and parameters for our fictional world and we must hold true to those rules. Those rules will serve as a sort of litmus test when considering the actions of our characters and the elements of the setting. It is the consistency of the story's components that makes it work.
Character assessments are critical. Each writer has their own way of learning about the characters in their story. Some may draw character webs. Others may interview the characters. The important thing is to use the information as building blocks during development and as a reminder of the character's attributes as the story progresses. For example: If I'm writing about a middle aged woman named Rose who is an activist with the anti-gun lobby and the only witness to a mob hit, I'm probably not going to have her choose to carry a gun. Why? Because I'm not sure that's believable. Would Rose be comfortable toting a Glock in her handbag? Would she know how to load the weapon? Or how to use it? Maybe not. However, I might give Rose a can of pepper spray. She may choose to wear a pair of good running shoes or hire a body guard.
We can apply the same reasoning to our imaginary places. If the leaves of a great oak tree are purple then they should stay purple. If an ogre is immune to magic, than no matter how tempting, the ogre should stay immune.
By obeying the rules we've set, we enable our story to progress. Ignoring the rules will cause our stories to stray into the unbelievable.