[spoilers ahead]
The biggest takeaway from the book was the fact that nothing changed for Rachel after Tom was proven guilty. Though the book had three separate protagonists from time to time and it wasn't possible to show that in the movie, one might only expect the film adaptation to stay true to the main conflict. The film whole heartedly became a story of Mira Kapoor which wasn't the case for Rachel Watson. Rachel was never the heroine of the story, the book never happened to centre it around here, she was bad in her own ways, and her presence was a doom to Tom at some point, the book somehow even defended Tom by giving him a stand point to an extent, that was the whole beauty of the book. That it had no hero, no villian to call it's own. And the adaptation messed up with that very beauty. So instead of calling it an adaptation, we better call it inspired by the novel by Paula Hawkins. Very disappointed.