I can't be the only one who walked out of the theater feeling like the movie was a completely different creature, right? The book is this intensely introspective, psychological horror crawl about obsession, self-destruction, and the deep unease of confronting the unknowable. The 'Area X' on page feels like a character itself, malevolent and intelligent, warping minds and memory. The movie ditches a ton of that internal monologue for stunning, terrifying visuals like the shimmer and that... bear thing. The Biologist in the book is a closed-off, alienated person studying her environment, while Portman's character is driven by her husband's disappearance. That changes everything. The endings are galaxies apart, too. The book leaves you with this profound, chilling ambiguity about what the Biologist has become after merging with the Crawler. The movie gives you that psychedelic lighthouse climax and the doppelgänger embrace, which is more of a direct, albeit weird, spectacle about self-destruction and change. I liked both, but for totally different reasons. The book messed with my head for weeks; the movie messed with my eyes for a few hours.
Honestly, the biggest plot shift is the tower/tunnel. In the book, the Biologist is drawn to a mysterious, living tower descending into the ground, with writing on the walls. The movie replaces it entirely with the lighthouse as the focal point. That one change ripples through the whole structure of the journey. The movie's version feels more like a straight line to a destination, while the book's exploration of the tower is a spiraling descent into madness. I still think about the phrasing 'Where lies the strangling fruit...' from the book. That never made it to screen, and it's a huge loss for the atmosphere.