People who have read the book...

I don't know that my response can provide exactly what you're looking for, as I only saw the movie once (shortly after it was first released). But the book is one of my favorite novels, which I have re-read over the years more times than I can count.

And what I remember thinking after watching the movie was that it didn't really go into enough detail with any of the key characters (e.g., Tom, Annie, Grace, or Robert) to fully understand or appreciate their roles in this story. The novel does a thorough job of exploring both the current story and how it relates to their pasts, without ever becoming overwhelming or boring. Also, although the novel is written in third person, it shifts throughout the story in whose perspective it primarily focuses on. So, as different events are occurring, they are described within the context of the thoughts, feelings, interpretations, and background knowledge of whoever is the author's choice of primary character at the time. Sometimes this character is someone directly involved in the event (e.g., Annie or Tom during their time alone with each other), and sometimes it's someone observing the primary people involved (e.g., Grace when she first realizes Annie and Tom have been together). I don't want to say too much, in case you ever decide that you want to try reading the novel. But, yes, I do think that the novel provides far more about Grace's perspective (e.g., regarding her disability and her feelings about Annie and Tom, first individualy and later as a couple when she eventually realizes that they have been together) than the movie.

I really wasn't very satisfied with the movie. It just felt like too much was missing (not so much with events, but with how the characters interpretted the events). But I've long thought that the novel tells a very compelling story, that has never left me wishing afterward for more info to put it into context. If you found the movie at all interesting, I would highly recommend the novel.