With a book, we visualize a scene based on the author’s description. Our imagination fills in the gaps as we correlate the words with our own experiences. My idea of what a character looks and sounds like can differ from someone else’s. In a movie, we all see and hear the same thing. A description in a book takes longer to visualize whereas visualizing something in a movie is instantaneous. A book can describe sound, a movie can produce sound. Most movies add music to enhance the experience.
I like to think of books and movies as separate versions of a core story, each offering something unique. This is not to say that it always works. Some stories lend themselves better to one form than the other. An episode of the Three Stooges comes to mind. I do, however, appreciate an effort to match the author’s vision. This requires that those making the movie understand or care about that vision. Movies are literary adaptations. They are taken out of their literary form and transformed into a dramatic work. The process of how this happens is best explained by Brad Fox, a film and television producer from Toronto Canada. He addresses the issue in an online post in Quora. He begins by explaining it this way:
The "typical" situation is that an author likely has little to no editorial input into how their work is adapted. Usually a producer (or studio) will “option” a book - ie: pay an author a smaller amount to have exclusive rights to try and prepare, finance, and create a feature film within a certain time period (usually several years).
He goes on to say:
Since most fiction/non-fiction authors are not screenwriters with track records, once an option is obtained, the producer/studio’s first job is usually to bring on a screenwriter to make an adaptation. Most quality screenwriters with track records (who would increase the odds of a film being financeable / filmable) won’t work on adaptation projects where the original author has significant control.
The reason for this, according to Brad, is “it just makes the process of trying to finance and create a movie too onerous for most to even attempt.”
In most cases, a book is written by one person. The vision is his/hers alone. With movies, Brad describes it this way, “film is a collaborative medium involving thousands of people (and millions of dollars) and each requiring many concessions to any individual ‘vision’ of the end result (including even the producers and director).”
The complete post by Brad Fox can be found on Quora.