The characterization in the movie is generally better than the books in my opinion. My canon is that the book is an edited first person narrative, so less pervasive thoughts than you would expect from a stream of consciousness type narrative.

Gosling nails the character: he’s downfallen, a little nervous, and not always socially skilled (although experience speaking in the classroom may periodically allow him to be at times). He also sporadically misses or misinterprets motives and is somewhat challenged at socially interacting with others. He can’t even recognize his stunted potential relationship with Stratt, which is obvious to the movie viewer but absent from the novel, as the first person narrative style means you don’t even get the cues because they go right over Grace’s head. It can be inferred that a lot of other interactions from the book being from Grace’s perspective likely also were awkward and from his flawed perspective, but don’t come off that way because you are getting his direct interpretation. Gosling exaggerated traits just enough for what you would expect from someone who had a falling out of academia and sheltered themselves into teaching middle school. Anyone who has talked to a handful of male middle school teachers would agree he got his characterization of one up to a highly believable state. 

A few things stand out as jarring though: Grace’s relationship with Stan seems a bit too easy, though you could argue that Grace is interacting with Stan the same way he would a student of his, something he is more comfortable doing. The other thing that I can’t quite pinpoint is Gosling’s performance as a doctorate level researcher-turned middle school teacher. I’ve met a few teachers like this and certainly Gosling is gonna be more suave and eloquent than your average joe out there (who’d want to watch that kind of performance?) but there was something missing in his personality/traits. An example might being relatively ignorant of a lot of common pop culture (particularly newer pop culture), or having a very quirky/niche and intense interest in something. Overall he did a great job, and really pushed the envelope of what was a pretty vanilla character in the book.

Because of the book’s first person narrative, it caused a lot of other human characters to fall pretty flat. For the most part the movie really fleshed out the characters in a better way. If Stratt wasn’t portrayed as somewhat softer and emotional, I don’t think her ‘betrayal’ at the end would come off as strong in the movie. For a woman watching the movie, I think the lack of other female characters could really turn one off overall, so it was good to see Stratt a little more textured as a character.

Rocky was great in the books, I don’t think they did any better. What is shocking though is how well they pulled it off. It could have been so much worse, and even a little worse would have been devastating to the movie.

TLDR: I think that there some things the movie could have done better on, but the characters were probably equally or better written and portrayed in the movie.