If you write a screenplay, or even a basic story… REGISTER IT WITH THE WGA. As your project progresses you can always update the registration. When you finish it before delivery Copyright it with the LIbrary of Congress. But, in the meantime get it registered with the WGA. You can do it at WGA West on line for $25. If you ever get into a credit grab situation you’ll be thankful the WGA has a copy of YOUR script. When a movie gets made your script seldom lives through the process… The producers start making changes, the director makes changes, the production company will want changes or make them, the exec producers will want changes or make them, the sales people will want changes… they’ll bring in additional writers, directors and producers will re-write specific sections, they’ll add new characters, they’ll combine characters, scenes, add new lines, the actors make changes. So when it comes to who wrote it? In the real world the WGA gets involved and determines who actually wrote the script based on a lot of data, including your version, the version as shot, the version as delivered, the changes made by others, the input made by others… I advise doing some research and you’ll find out just how often this happens… Every one of my scripts are registered with the WGA and I may do 3 or four updates in that registration as the script develops. I also send my final script for copyright to cover other legal issues that might arise.