I have a great idea. Can I copyright it?
Copyright protects the expression of the idea, but not the ideas ideas themselves. This is easier to understand if you remember the goals of our Founding Fathers -- to reward creations, but protect the free flow of ideas and information. For example (this is Plato's explanation, the Plantonic Ideal, of the concept long before copyright), if I ask you what a chair is, you get a picture in your head. The picture I get in my head is different; the picture Buffy gets ino her head is different. These are the "ideas" of what a chair is. However, if you draw the chair in your head or use words to describe the chair, that is the "expression" of the idea and that is protected by copyright.
Another example, suppose you had an idea for a movie about an African prince who comes to the U.S. to find a bride, and you wrote it into a story outline. The written story would qualify for copyright protection. However, under copyright, there is nothing to prevent another author from using that same idea to do his or her own movie script. To protect your "idea" you could insist upon entering into a confidentiality agreement before disclosing the idea to anyone, thus protecting your idea as a "trade secret".
This very thing happened to Art Buchwald, the humorist. Buchwald presented his story outline about an African prince to a movie studio. Then the studio ripped it off and produced "Coming to America" with Eddie Murphy. Buchwald sued the studio for breach of its agreement with him. The court found that Buchwalds idea was protected through the confidentiality agreement he was smart enough to insist on before disclosing the idea to the studio. If it can happen to someone famous, it can also happen to you.