Gather reviews of the film from your school library and categorize the arguments that they give about the movie. If you disagree with any of the arguments, then select one argument or a set of arguments and and let your counter-argument serve as your thesis. If your thesis repeats a point given in one of the reviews, look for other things about the film, its background, or its reception that's not developed by the reviews and that support your thesis.
Make sure to try these first before approaching your professor and asking for advice. Don't forget to bring your research materials and any notes (including outlines and drafts).
