'Red, White & Royal Blue' Director Explains Book-to-Movie Changes, Why Some Things 'Had to Go' (Exclusive)

Casey McQuiston's 2019 bestseller is now a movie from director Matthew López, now on Prime Video

Turning a beloved bestseller with over 400 pages of rom-com fireworks into a two-hour movie is no small feat.

"As the director of a movie, you have a very different responsibility than the author of a book," Matthew López, who also co-wrote the Red, White & Royal Blue screenplay with Ted Malawer, tells PEOPLE. "My attitude about it is however long the audiobook is, subtract that by two hours and that's what's not in the movie."

Casey McQuiston's 2019 novel, like the new movie, centers on a son of the U.S. president Alex Claremont-Diaz (Taylor Zakhar Perez) falling in love with Prince Henry of England (Nicholas Galitzine).

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There are significant changes from the book to movie. For example, Alex's sister June is removed, and his parents, President Ellen Claremont and Oscar Diaz (Uma Thurman and Clifton Collins Jr.) are portrayed as together rather than divorced. Plus Henry's grandmother Queen Mary is swapped for a grandfather, King James III (played by Stephen Fry).

Other subplots are excised or condensed, but the central love story remains intact.

"Anything that wasn't about Alex and Henry didn't belong in the film. Anything that didn't feed into that story had to go. It's just the logic of movie storytelling," says López. "There's actually less than 20 seconds of the film in which one or the other doesn't appear."

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The director continues, "I was making decisions that maybe I'd have to explain to the fans later on, but at the end of the day, I knew what I was doing was in the best interest of the film. I am one of the fans, and I knew the best way to deliver something fans would appreciate is to make sure I delivered a good film."

"When I realized that was my mission and responsibility, everything became clear in terms of decision-making," he says.

Red, White & Royal Blue author McQuiston clearly approves of the film adaptation, writing on Instagram earlier this week that the book has now "become much bigger than me — something gorgeous and perfect in its own way, beaming out into a bigger world."

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The actors felt the pressure of doing the popular book justice too.

Says Perez, "We have a lot on our shoulders and we had to make sure this is a really good film — or else we are going down!" Galitzine adds, "It was very cool, if not a little nerve-wracking, because most of the time you're not doing something which has this built-in fan base. And a really fervent, impassioned fan base at that."

Red, White & Royal Blue is now streaming on Prime Video. The novel is available wherever books are sold.

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