It is very common for a novel that is turned into a movie to have some differences. The novel and film Persepolis, do have little differences but I think the overall purpose is the same. One small difference is the starting of the novel and film. The novel is in chronological order starting right from Marji childhood. In the movie, the opening scene is of adult Marji sitting in an airport in Paris. While sitting in the airport she begins to have flashbacks of her past and childhood. This new scene may have been added to make the film more interesting. Something to notice is the use of color. In the graphic novel, Satrapi mostly uses black and white while using grey very occasionally. In the film, we see some color in the present scenes of adult
