It completely depends on the director's vision and the medium's needs. In 'The Notebook', for instance, the movie amplified the class conflict between Noah and Allie, making it more visual and immediate through setting and costume. The book's internal monologue about social pressure gets translated into those tense dinner scenes with her parents.
But sometimes, the compression hurts. A novel can build a relationship over hundreds of pages with small, quiet moments. A two-hour film has to pick a few key scenes, which can make a connection feel more like a series of dramatic highlights than a gradual unfolding. I think that's why some adaptations, like 'Pride & Prejudice' (2005), focus so much on loaded glances and silences—they're trying to convey the book's subtext visually, but it shifts the dynamic from Elizabeth's sharp internal observations to a more physical, almost wordless tension between the actors.
Some changes are just inevitable, like aging up teen characters for older actors, which totally alters the 'coming-of-age' innocence of the original dynamic.