Mainly, we're all McLuhans now. We take it for granted that the contents of each new medium, the world wide web for example, is the media that preceded it. In the case of the web, television, film, photography, books and magazines of all kinds make up a substantial part of its contents.
It seems to me the web has, up to now, functioned mainly as a mass distribution medium. The content of the web, a photograph for instance, may be transformed by being published in the context of the web, where it is available to so many people so fast and collides with so much other information, but it is not altered on purpose to make it "webic" in the way books and plays are altered to make them "filmic." On the way to becoming a film, a book is broken down, then put together again as a screenplay and a film.
There are several ways to use a book to produce a screenplay. The easiest way, probably, is to ignore the book's characters and plot and recreate the "essence" of the book in film. Warhol's Vinyl, for example, captures the essence of A Clockwork Orange, without burdening the film with Burgess's characters and plot. That's probably not the most commercially successful way to turn a book into a film.
The commercially successful way, the Hollywood way, is to respect the narrative and characters of the novel and to recreate those elements with acting, cinematography, sound and editing in a way that "brings the novel to life." The quality of the novel, of its characters and plot, matter. If necessary, the film may deviate from the novel, but changes to the original are made in the spirit of improvement. Kubrick was faithful to Burgess's plot in his film version of A Clockwork Orange. Ridley Scott, on the other hand, probably intended Blade Runner to be an improved version of Dick's Do Androids Dream Of Electric Sheep. His departures from Dick's narrative and characters were intended to produce a better, more successful story.
The third way, the auteur way, is to use the novel's characters and plot simply as a place to start. French New Wave directors bought the rights to dime store novels for their plots. Almost any plot would do, because the films they made weren't about the narrative. The story was beside the point. Just something to hang the film on. Art is synthesized experience. For film makers like Goddard, the story was just an occasion for that synthesis.