If the title of this post causes confusion, I will clarify. I refer to books which are made into movies, by moralists--moralists who depart from the book on which the movie is said to be based, for what they consider to be the good of the audience, or because the book is lacking in some respect which the moralist believes must be corrected in the movie.
Most recently, we see this take place in Peter Jackson's prolonged treatment of a relatively short book by J.R.R. Tolkien, The Hobbit. Compared to The Lord of the Rings which followed it, and which rather obviously addressed good and evil, I've always thought The Hobbit to have been written with no moral point in mind, rather like other classic "children's" works, before we began writing books to teach children what we refrain from teaching them ourselves. In the film versions, it becomes a kind of morality tale regarding the perils of avarice.
Then there is the insertion of characters and events; something at which Mr. Jackson excels, and indulged in even when filming The Lord of the Rings. For example, he managed to coax an entire host of Elves to come to the relief of Rohan at Helm's Deep, though no such thing took place in the book. The chief of this host, who looked disturbingly like Legolas as played by Orlando Bloom, as did the members of the host we could see, announced they came to stand beside Men in the fight against evil. Most uplifting. But I wonder whether Jackson unwittingly was suggesting all Elves look the same, to him.
This time, though, he inserts as a major character one not mentioned in the book at all, an Elf woman of his own devising. From what I've read he did this because he thought girls should have as kind of role model someone of their own, though an Elf, slaughtering Orcs and Goblins. Not content with manufacturing this character, he conjures a romantic relation between her and one of the dwarves, probably to show us that we can all love one another and are all really the same, regardless of our differences.
I find myself wondering what it is that possesses someone making a movie based on a book, to depart from the book radically. Presumably, the book is being made into a movie because the book is loved and admired, or at least very popular. Why, then, change the book?
It's possible, of course, that scenes from a book cannot be effectively presented in a movie. Our technology allows us to do a great deal in movies we couldn't do before, of course. But even so, sometimes a picture won't do what was done in print.
It's also possible, of course, that a movie may be better than a book by departing from it, at least as far as lesser books are concerned, or that a movie may depart from a book in such a way as to take its place as a uniquely separate work of art. Stanley Kubrick made a habit of taking books and running with them, in directions the author did not or could not imagine.
Although Kubrick collaborated with Arthur C. Clarke in making 2001: A Space Odyssey, the movie and the book differ significantly. There, though, Clarke may have written the book after the movie. This has never been clear to me. And Stephen King was appalled to find out what Kubrick had done to The Shining, though frankly I would take the movie over the book any day. I consider the movie far superior. As if to prove this to be the case, King made his own movie version of his book, which was uninteresting. I'm not sure of what Anthony Burgess thought of A Clockwork Orange, but it's justly considered a great movie.
"Better than the book" doesn't work in the case of Mr. Jackson's adaptions of Tolkien, in my opinion, but neither are they such as to be considered unique works in and of themselves, being too derivative except in certain carefully selected ways. So are others where moralists make movies. The film version of The Scarlett Letter has Hester running off with Dimmesdale, presumably to live happily ever after. Disney, of course, annihilated The Hunchback of Notre Dame, with everyone loving and accepting Quasimodo. Such are perversions of great novels.
One can go on, of course. But I think it takes an especially arrogant and self-righteous person to radically change a book, "for the better." Great books should be taken "as is" and with all faults. They cannot be made "better" and if those making movies seek to accomplish such a thing they should leave well enough alone.