What Copyright DOES NOT Protect?
Copyright does not protect the idea itself, in fact, ideas are as “free as the wind.” There is a “Common law copyright” in unfixed ideas, and this subsists in the expression of the ideas on “canvas.” Once fixed in a tangible medium the protections and evidentiary effects of the Copyright Act applies in the USA.

What Copyright DOES Protect?
Copyright protects the original expression of the ideas. In copyright law there exists in logic the “IDEA-EXPRESSION DICHOTOMY”, or THE ”PROCESS-EXPRESSION, METHOD-EXPRESSION, AND USEFUL-EXPRESSION distinctions.” This seems esoteric but it is saying that since Copyright does NOT protect the idea, but only the “fixed” expression of the idea, when someone “copies” an idea, there is usually limited if any copyright protection for that “copying”. If, however, one copies the expression as fixed in some tangible form, there can be infringement
liability because there is a “copyright” on the “fixed” original expression to copy. Whether or not liability may exist, the copyright owner cannot enforce it court unless it is registered.
“Whether or not liability may exist, the copyright owner cannot enforce it court unless it is registered.”

​For example if Doe tells a film Producer (the money behind a film) friend of his about his idea for a great film (assuming the Producer will listen). Later Doe watches a film that he swears was his idea. He needs to prove it was “his”: a copy of the script or treatment for the screenplay he wrote, or a book he wrote containing these ideas or other form of tangible copyrightable matter to show that he indeed wrote it and to claim for royalties or an injunction. Doe also must register it or he cannot go to court to sue for the infringement. That is not the end of the story but an example of the dichotomy.