Ethan was portrayed in the movie with a lot of anger toward Anna, but the book painted a darker picture of the lengths Ethan went with his obsession.
In confidence, Anna told GrannyLizzie that she and her husband Ed always greeted each other by saying “Guess who?” leading Ethan to create the “guesswhoanna” gmail address.
The book played out this insider knowledge to be more proof that Anna was concocting the whole situation in her head, while in the movie it was just creepy without explanation.
Not only did he break her cat’s leg (which was never explicitly stated in the movie), he also manipulated her in small ways like changing the PIN on her cellphone.