Movie Making Manual/Legal/Protecting Ideas
It is not possible to copyright an idea - you only get copyright in the expression of the idea eg the script, the film or even the synopsis.
Screenwriters can sometimes be overly paranoid that someone else will steal their ideas. The U.S. Copyright Office offers one way to ensure your copyrights are protected for life, for a fee $45. Another way is to register your script with a script registration service, like the Writer's Guild of America Registry.
Depending on your country of origin, there is also the poor man's copyright. This action of mailing your material to yourself is not necessarily admissible in a United States court of law, as it is easily spoofed; however, in many European countries it may or may not be admissible, but will most likely be accepted with some form of "recorded delivery", as opposed to regular mail. In any case, if you fear your intellectual property may be stolen, it is well worth the money to register it with local copyright officials.
Another suitable means of protecting your work is to use a notary service (available at most banks). You can have the notary date and sign your documents to serve as a proof if your work is ever stolen.
See the section on Copyright for more information.