You may develop a screenplay based on a public domain work, such as a play by Shakespeare. A public domain work may be used by anyone in any way without the need for permission or payment to the author or the author’s estate. A work is in the public domain when the work lacks copyright…

You are free to independently create works that may be similar to others, as long as you can prove that they were, indeed, completely original to you. The U.S. Copyright Act grants the author of literary property such as a book, video game, screenplay, motion picture, play, etc., the exclusive right to use his creation….