If you make unauthorized changes to a script that is covered by copyright, you are indeed violating the author's rights under copyright.
Under copyright, the play belongs to the author. Changes cannot be made without the author's permission.
Most contracts granting the right to perform a play state this explicitly, but even if they did not, statute would protect the author against this kind of infringement.
Did you really think that if you pay royalties, you had the right to rewrite, cut, or do whatever you want with the script? |