What is the “automatic difference” that Robert Stam describes between text and film? In other words, why is it impossible to have a “faithful” adaptation?

In Beyond Fidelity: The Dialogics of Adaptation, the American film theorist Robert Stam questions the very possibility of a faithful adaptation of literature through the medium of film. Some adaptations are undoubtedly more faithful than others, but complete fidelity is impossible, owing to what Stam calls the "automatic difference" between the respective media.
A film is not a book; its creation involves a number of different elements, such as sound and lighting, which find no parallels with the written word. That being the case, the traditional notion of fidelity as it relates to cinematic adaptations of literary works must be challenged. That's not to say that the question of fidelity is completely irrelevant; some films are more adept at capturing the essence of a book than others or are more true to the most important components and fundamentals of a novel. But complete fidelity is a chimera, for the reasons we've already discussed.