Cramming the content of a novel into a two-hour movie means you're going to see characters eliminated and scenes combined, that's just a given. To say "the book was better" is a well-worn cliche, so I won't use it here, but having read the book first, I was saddled with expectations that were never met.
For a light, two-hour "Hallmark movie the week" style flick? Red, White & Royal Blue was fine. But the cost was having to eliminate most of the emotion, peril and building up to the big scenes that the book had. The pacing felt rushed, and it seemed like the director was trying to get to from one big moment from the book to the next, as quickly as possible before they ran out of time.
Casting-wise, the actors all pretty much looked exactly like how they were meticulously described in the book-with one glaring exception. In the book, Alex was much younger. Closer to 21. He was also noticeably shorter than Henry too, at around 5' 7" or 5' 8" (which is why his "He doesn't look 6' 2"!" line in the film seems to come out of nowhere). Nicholas Galitzine was a decade to old and far too tall for the part. Instead of seeing a young college student fall for a tall, handsome prince? We spent two hours watching two grown Abercrombie & Fitch models trying to "out hot" each other.
Overall the film was cute and worked fine, but it lacked most of the emotion and actual peril the book rewarded us with-and what a 3 to 5-episode mini-series could've delivered.