If you're writing about disabled characters and you are not disabled, develop these characters beyond their abilities to tell an authentic story. Consider a disability part of a character's makeup, not the driving engine for your screenplay.
Great screenplay writing has strongly defined characters that attract financiers, studios, actors, and directors. Wendy Kram gives advice on creating memorable characters.
Characters are engines that drive ideas into a story. Each of the major character types plays a specific role in that process. Michael Tabb highlights THE MENTOR.