After finishing the Project Hail Mary novel after seeing the movie, I have a few thoughts on the changes the movie made.
Stuff the Movie Added That I Liked More Than the Book:
Carl. Since the book is in first-person, the narration in the book is basically Grace's stream-of-consciousness of his thought process. But in a film, having him constantly narrating his thought process for the audience would be weird, so the addition of Carl as someone to bounce ideas off of worked to explain the science experiments more naturally.
Rocky having an actual text-to-speech voice. In the book, Grace just catalogued Eridian words in a database and would constantly refer to it while he became more proficient at just understanding it himself, and Rocky's dialogue was italicized to indicate it was a translation, but not necessarily a spoken translation. Again, this was for streamlining in a film medium.
Grace's nerdy shirts. In the book, Grace didn't have any personal effects packed on the ship, they were all still for DuBois.
Not an addition, but the movie moving Rocky's offer of extra fuel to the third act. In the book, he makes the offer almost immediately after moving onto Grace's ship rather than right before their climactic "fishing" expedition. I feel like this scene happening so early in the book makes it lose some of its weight.
Grace actually getting to go to Rocky's ship. In the book it was established that the temperature and pressure of the Eridian environment would kill him, so there was no way, and he was bummed, but it was just left at that.
Stuff the Movie Cut Out That I'm Honestly Okay With:
The panspermia plotline. I saw that it actually was brought up in a deleted scene, but I'm glad they didn't go with it. For a story that otherwise is all about rigorous scientific validation, and then just going "Wow, what are the odds we have this biological similarity? We must all be descended from Astrophage seeding life throughout the universe!" and then just settling on that with zero rigor feels counter to the overall theme.
The entire Taumoeba breeding fiasco. Similar to my complaint above about Rocky's offer of fuel coming too early in the book, the fishing expedition and Rocky nearly dying is the climax of the story, but there's still 1/3 of the book left at that point. And the fact that they do so much dicking around with breeding nitrogen-resistant Taumoeba, it getting loose on the Hail Mary and Grace having to purge and sterilize all the fuel tanks, then having to rig the Beetles to the ship as makeshift engines to limp back to Rocky's ship just kind of kills the tension of that earlier moment. Also, it sets up the Beetles as way too blatant of a Chekhov's Gun, what with the repeated pointing out that "I saved one Beetle just in case I might need it for something later, HINT HINT".
Grace's food situation at the end. It's fine to just surmise that they've synthesized nutrients for him on Erid without specifying that he's eating his own cloned muscle tissue. I feel like that would have been tonally jarring for the movie to bring up specifically.
Stuff the Movie Added That I Wish it Didn't:
The attempts to soften Stratt, or even try to hint at some chemistry between her and Grace. Like, yeah, it wasn't much, but none of that stuff like the karaoke scene were in the book, and where Stratt was 100% no-nonsense through and through. She was basically just dragging Grace along because she'd pegged him as the POV character and none of her actions would get recorded for posterity if he wasn't there to observe (why else would a microbiologist be present at a copyright infringement hearing, if not so that Stratt could ensure that her being like "Yeah, I pirated all your software to save the world. What are you gonna do about it?" was recorded for all to see).
Making Yao a goofball, too. In the book, he was always described as super serious. He wanted a gun as his means of suicide so that he could take the other two out first in case their preferred methods were unsuccessful. But he was made silly right along with everyone else in the movie, with the "You make a funny face in literally every photo", which made the characters less differentiated.
Stuff the Movie Cut That I Wish it Didn't:
Grace having to get Rocky back to his habitat after Rocky saves him. In the book, Rocky is only able to save Grace from being crushed against the control panel by the G-forces before he collapses, and Grace has to drag him back to his habitat. But Rocky is like 400 pounds and molten hot, so Grace ends up suffering burns for his efforts.
Dimitri and his team of crazy Russian scientists. Like, he's in the movie, or at least a character in his role is, but he isn't named and has like two lines.
Various one-liners like:
"It took you two days to think of poking it with a stick?"
"Yes, always Earth units, you are bad at math."
"I scary space monster, you leaky space blob."
Subtitling Rocky with more fluid, casual English in the ending. The reason Rocky speaks in broken English for most of the movie is because his translation is limited to a few hundred English words. But by the end of the movie, both Rocky and Grace are fluent in each other's languages, so Rocky's speech style is appropriately more fluid, and he says stuff like, "I haven't seen that in a long-ass time!"
Questions I Had in the Movie That the Book Answered:
If Eridians can't detect light, how did they detect the Petrova Line, which is infrared light? A: They have sensors that can detect things that they can't physically sense, too
Okay, if the Eridians can detect light through instruments, why hadn't they discovered radiation? A: Erid has a massive magnetosphere that protects the planet from radiation that extends farther out than Eridians had ever traveled, so they'd never seen it even on their instruments
Why did the Astrophage protect Rocky from radiation? A: Astrophage are black bodies that absorb all energy that come into contact with them. The Hail Mary has a layer of Astrophage throughout the hull for this purpose.
Why didn't they just pull out of Adrian's atmosphere first before going out to reel in the collector? A: The drag on the collector from the atmosphere while traveling at escape velocity would have vaporized it.
How did Grace find Rocky's ship when he went back for him? A: By doing space donuts to use his Petrova frequency engines and Petrovascope as a makeshift long-distance radar.
Questions I Had in the Movie That the Book Contemplated for One Sentence and then Dropped it Because it Would Destroy the Entire Premise:
What happens if Astrophage comes into contact with a star that has no planets, or at least none with a CO2 atmosphere for it to breed in? Doesn't the fact that literally every neighboring star has been infected to the point of dimming (meaning the Astrophage is multiplying) necessarily imply that every star has a CO2-harboring planet? How likely is that? In the book, Grace worries that the reason Tau Ceti isn't dimming is simply because it doesn't have a planet for Astrophage to breed on, in which case the entire mission would have been for nothing because that's not something they can fix (short of blowing up Venus, which I wouldn't put it past Stratt to try). Thankfully that turned out to not be the case, but realistically, wouldn't that be the most likely scenario?