From idea to screen, what is copyrightable?

American copyright law protects “the expression of an author”, a concept that is hard to define. It primarily prevents verbatim reproductions of an author’s words, but it can also encompass finely drawn characters and detailed plot points as well as narrative progression. Yet there is no clear ruling as to how many elements of an author’s work need to be mirrored before an accusation of copyright violation holds weight.

More here.