Prose and film are very different mediums, and each medium works best with different types of characters.
The movie gives Grace new traits and quirks in order to flesh out his character and streamline his character arc into something dramatic that can be clearly conveyed through motion. Most of us here, myself included, watched the movie first and accepted these traits as the key parts of the character, and continued to apply them to the book version of Grace, even though he is written in a significantly different way.
Here's the biggest one. In the book, Grace's refusal of the call scene and the immediate follow-up use the word "coward" five times within a few pages. The word isn't used in the book anywhere else, and the scene itself is one of the whole story's biggest surprise reveals both for the reader and the protagonist (the other big surprise reveal being the alien ship). Then, the movie takes the idea of cowardice from this scene and rebuilds it into Grace's primary and consistent character trait, and gives him an entirely new character arc about overcoming his fears and insecurities and learning to be brave. The three parallelled scenes in which Grace runs away — from Stratt, from the Blip-A, from the men with the syringe? Added in the movie. Dodging his students' uncomfortable questions, speaking into Stratt's ear instead of the microphone, cowering from the "bomb", backing out of his first spacewalk? All added in the movie. The Hollywood arc about an everyman learning how to be a hero straight up doesn't exist in the source material. In the book, the contrast between "past Grace who refused to sacrifice himself when asked" and "present Grace who sacrifices himself on his own volition" is specific to the third act, his overall arc is more concerned with identity and behavior being shaped by perception rather than heroism being an internal trait that needs to be cultivated, and insecurity isn't present at all.
Grace lacking connection to anyone on the Petrova Taskforce except Stratt, looking like an awkward misfit outsider among them? The photo where he's alone, the teacher-speak that weirds others out, the karaoke bar where nobody wants to hang out with him? All movie-only. In the book, he asks a guy out for a beer in the middle of his second meeting on the Vat, he calls the crew "friends", and they request him specifically to deal with a sensitive matter.
His impulsive physical aggression? Throwing the waste bin, hitting the spectrometer, punching the monitor? None of that was in the book. Insulting a scholar at a conference? In the book, he burned his bridges via a published paper, which is very far from a momentary outburst.
It's not like these traits were invented wholesale for the movie, and book Grace was never scared or lonely or angry. He dreads the loss of artificial gravity so much he fails to prepare for it, he "turns to flee" when he sees Rocky's spiderlike figure for the first time, he doesn't realize his coworkers like him despite leaving evidence to the contrary for the reader, he admits he "didn’t have much of a social life back when things were normal", he immediately starts squabbling with Lokken upon meeting her, he snaps even at Rocky, he screams at Stratt until he's sore.
But the book and the movie have different goals and methods. In the book, plot, science and worldbuilding come first; it's hard sci-fi, not a work of psychological realism. The prose doesn't focus on the protagonist's emotions, but since he's the narrator, the structure and the style of the book themselves become parts of his characterization. He is the kind of person who isn't good at reading others and describing feelings; he is the kind of person whose mental space is largely taken up by facts and calculations, and who likes it that way; he is the kind of person who built up a habit of self-censoring even within the privacy of his own mind.
The movie, on the other hand, is a blockbuster starring an A-list actor and featuring an adorable puppet, targeted at audiences starting from the youngest ages. The music plays such a big part that the movie is halfway to being a ballet/opera/musical. Rocky's voice is part animalistic alien sounds and part English text-to-speech, so his body language becomes even more crucial to his characterization than it usually is for human characters. In this context, the protagonist's personality also needs to be conveyed through facial expressions and body language no less than through the dialogue, and perhaps even more. Which is why the movie gives him all these emotional, visceral reactions that are expressed through moving or positioning his body a certain way. It's also why frequent crying is a trait that's equally prominent in the book and the movie — it works well in both mediums.
All three main characters are to some extent different between the book and the movie. When we make headcanons and fanworks, that doesn't matter — like everyone else, some things I liked better in the book and some I liked better in the movie, and I expect artists and writers to use both canons as the jumping-off point just like the movie used the book as its jumping-off point. But when it comes to analyzing the characters as they are in the source material, the distinction is I think more important, because half of their characterization exists only in the book but not the movie, or vice versa.
