One way to protect a common law copyright is simply to mail the ms. to yourself, registered, so that the date is clearly stamped. Do not open it, once it arrives. Keep it sealed, preferably in a safe deposit box at the bank. If, in the future, there is any dispute about who had the original idea, you have the evidence in the form of a dated piece of mail. In your case, since you're not writing the actual script, you would write a synopsis of your basic idea.