Filmmakers have such a tricky balance. I'm always skeptical of adaptations because so much of a novel's depth is internal – thoughts, feelings, the texture of a character's mind. A good film can't just rely on voiceover, which often feels lazy. It's in the actor's eyes, the way they hold themselves in a silent moment. Think of how 'The Godfather' showed Michael Corleone's chilling transformation mostly through Al Pacino's performance in scenes with almost no dialogue. The novel gives you his thoughts, but the film makes you feel his isolation through framing and shadow. The depth isn't replicated; it's translated into a visual and physical language. It requires everyone – director, actor, cinematographer – to be reading from the same page, literally and figuratively.
Sometimes they have to invent new scenes to show what the book told. The film of 'The Silence of the Lambs' added that brief moment where Clarice Starling flinches at a man's loud laugh in the elevator – it wasn't in the book, but it visually communicated her hyper-vigilance and trauma better than paragraphs of description ever could. That's preservation, but through creative reinvention. Not every filmmaker gets that.