What copyright laws should I look out for?

If the book's author has been dead for more than 50 years, it's in the public domain and you can do what you please with it. Films are much less clear-cut though; it's tied up with the screenplay author's date of death as well as the year the film was made, and there's more to it as well because rights can be held for any number of years after death. You will have to check this out very, very carefully because you will be sued if you don't get permission! Audience size is totally irrelevant to whether copyright has been breached or not.