Great writers aren't limited by genre
I've just seen a posting on Screenwriting Staffing from someone looking for a screenwriter to doctor a horror film script. This person will only entertain screenwriters who are passionate about the horror genre, and/or have more than one horror screenplay credit. Why? A good screenwriter doesn't need to specialise in any genre. The same screenwriting instincts (story, structure, narrative) and screenwriting techniques (nuanced, dynamic screenplay characters, ellipsis, enter s