Films are often used as teaching tools to provide variety of media or an alternate perspective. Reaction papers are used to ensure that students understand the material and see how it fits within the context of the course in which it was shown. Unless it is part of the assignment, you don't need to give a plot summary or provide your opinion. Instead, focus on the movie as literature, or how it fits into the context of what you are learning.
Treat the film as literature in an English class. Write about how the themes and motifs come across in the course of the movie. Write about characterization and dialogue, especially if you are watching a film version of a novel. Compare the novel to the film in terms of story and style, and how the portrayal of characters met your expectations or surprised you. Give your opinion of how the form impacted the story -- does the story work better as a book or a movie? How are the two different?
Select the appropriate focus for your paper. Writing about a film in a history class could entail identifying where the film is historically accurate and where it takes artistic license. If you felt more connected to the characters than you would if you had read about them in a textbook, explain why. Talk about how the characters are portrayed and who is the most sympathetic. Did this meet your expectations, based on your knowledge of history, or did it surprise you? Talk about how the historical period is portrayed. Include elements of the film that tipped you off as to the period in which is was made.
Determine what elements to include. Write in a film class about how you think the production elements worked to tell the story. Was it effective? How could it have been better or worse? Differentiate between the screen writing, the cinematography and the acting. Were all of these elements working in harmony? Is it typical of this director's body of work? What about the film was predictable or surprising?
Review your essay after it is completed to be sure you have included enough detail to explain your thinking and your language is clear. Examine punctuation, spelling and format for consistency since mechanical errors make your ideas more difficult to understand.