Or take Tarkovsky: both Stalker and Solaris are based on books, but only loosely. Stalker, for example, was a short sci-fi / horror story, if I remember correctly, and in the movie this is just a remote backstory, setting the scene to more serious topics and mysticism.
Stalker is based on Roadside Picnic, basically a scifi novella. Tarkovsky heavily condenses the story to its most basic structure, the story itself is actually a masterpiece in understated commentary on society, class, individual will, and the human response to the unknown. All of this shows up in Tarkovsky’s film albeit in very sparse and intentional ways. As a filmmaker myself, I have realized that to adapt a story into a film it must be radically altered in order to fit the language of cinema because to merely transfer the plot, etc, starts to be more like theater than cinema
