Let's take fairies, for example. My current work in progress has some fairies in it. My deleima becomes how to portray the fairies. Do I use the popular "Tinkerbell" model? Or do I go more traditional with the belief that not all fairies are good or cute or sparkly?
It would be easy to go the Tink route. Who hasn't heard of Tinkerbell and her friends? Fearless, determined, a little bit spoiled -- that would be easy to write. Add in that they are cute and not a bit scary, and why not pick them?
What about a species of fairy covered in course brown hair who love to help, but aren't all that smart? Sea monsters thought to be fairies? One that can only be seen right before you die? According to A Witch's Guide to Faery Folk by Edain McCoy those are just three of the real fairies.
Having never actually seen a fairy (that I know of), who's to say which idea is correct? And if I haven't seen a fairy, how will Cerri? Should it be a talent she has? Are there "spells" or "potions" that could help her?
When building a paranormal world (or a world where paranormal is a little more normal) it's important to make each scenario believable within that world. It requires the author to know what's what in that world and stick to the rules already established. I might not have all the answers, but in Cerri's world I have a pretty good idea of how everything works.
By the way, I've opted for a more traditional fairy.