If your students have read a book and watched its movie counterpart, why not have them participate in an online discussion rather than simply writing a paper alone? This printable sheet gives directions for students to have a meaningful, detailed online discussion in small groups.
In my own classroom, I have assigned students into small groups on Edmodo and given them a week to participate in their discussion with their assigned small group. However, you don't have to use Edmodo, or even small groups-- the questions here can help any student compare a book and its movie version in any situation. The document is in Word so that teachers can tweak and differentiate any part of the directions or rubric.
The rubric is included to ensure insightful participation and "netiquette." Online discussion is a staple in many college courses-- train your students now to practice these important skills!