If it's a good work of adaptation, the book should remain a book and the film should remain a film, and you should not necessarily read the book to see the film. If you do need that, then that means that it's a failure. That is what I think.
Adaptations are a unique art form that bridge two mediums—literature and cinema—each with its own vocabulary and strengths. When done thoughtfully, a film adaptation can enhance the original story, offering visual and auditory richness that complements the written word. However, as the quote suggests, a successful adaptation should stand independently from the source material. If viewers find themselves needing to read the book to fully understand or appreciate the film, it may imply that the adaptation has failed to capture the essence, flavor, or core messages of the original work. This perspective emphasizes respect for both mediums as distinct art forms with unique expressive capacities. A well-adapted film respects the source but also translates it into a language suitable for cinema, avoiding mere replication of scenes or dialogue. Conversely, an adaptation that relies heavily on viewers' familiarity with the book risks being superficial or incomplete. This raises interesting questions about the purpose of adaptations: should they aim to faithfully replicate the source, serve as a reinterpretation, or stand alone as independent works? Striking a balance between loyalty and creativity is crucial. When successful, film adaptations can introduce stories to a broader audience, inspire new interpretations, and add to the cultural conversation without diminishing the original. Overall, this quote underscores that adapting needs a level of mastery—where the medium is respected, and each form is allowed to thrive on its own terms—thus preserving the integrity of both the book and the film.