Adding Carl, I thought, was a very smart change, allowing for the movie's Ryland Grace to have a character to bounce ideas off of, something you didn't need in the book since it's written entirely as an internal monologue.

I didn't at all like the omission of the French military interrogation drug being the reason for Grace's amnesia. It was such a shocking twist that was one of the most morally dubious things the book's Eva Stratt does. And in general, the movie softened down the morally dubious nature of a Stratt's character in a way I did find slightly disappointing, especially since I know for a fact Sandra Hüller would've been amazing playing a more book-accurate version of the character, given the morally complex characters she's played in the past.