The film adaptation of Harry Potter and the Prisoner of Azkaban contains the worst page to screen change in the entire series. The change not only negatively impacts the moment in the film, but has serious repercussions for the entire series. When Harry and Hermione follow Ron and a mysterious black dog under the Whomping Willow and discover Sirius Black is an animagus, the book and the film handle the resulting confrontation differently.
In the book Ron stands on his broken leg, positions himself between Harry and Sirius and says that Sirius will have to kill him to get to Harry. He does not hesitate to put himself in harm's way to protect his best friend. This moment highlights his bravery and loyalty so perfectly that it is a real detriment to his character to have that moment taken away from him in the film for seemingly no reason.
Instead, in the film adaptation, the moment is given to Hermione. She is the one who stands between Sirius and Harry and informs the wanted criminal that he will have to kill her to get to Harry. In this version, Ron is literally sidelined and he doesn't get to demonstrate his bold bravery. This scene not only undermines Ron's character, but it also does Hermione a disservice. By plugging Hermione into this moment, her character becomes too flexible, making her embody whatever is needed in the moment more than a specific set of traits.
This change served as a dangerous first domino in a cascade of instances where book-Ron's lines and actions were given to movie-Hermione in a way that really undermined Ron's role in the story. Prisoner of Azkaban shifted the entire series in many ways, most of them good, like updating the aesthetic of the robes and school uniforms. But the change of sidelining Ron in favor of Hermione was the series' most unfortunate mistake.
Ron is Harry's best friend. Obviously, Harry, Ron and Hermione are a trio and all best friends, but in the end it comes down to Harry and Ron. In the book of Harry Potter and the Goblet of Fire, when Harry and Ron are fighting, Harry flat out expresses how much he values and cherishes Ron's friendship, with no disrespect to Hermione. Changing who defends Harry in the movie of Prisoner of Azkaban, starts to shift that Harry and Ron dynamic and throws Hermione and Harry together for moments and conversations that in the book belong to Ron and Harry.
This change was unfair to both characters, and ultimately, became a flaw in the film series as a whole. It removed dimension from brilliantly crafted and well-rounded individuals, leading Ron to become simply the comic relief. While Rupert Grint has excellent comedic timing, his talents could have added layers of complexity to Ron, making the character more than just a goofy sidekick. Grint has a great ability to express such specific emotions with simple facial expressions that it is a shame the films chose to overlook him so often. Grint likely would have done an amazing job delivering the line in the Shrieking Shack, as it would require him to be brave, scared, faint and very injured all at once for a very brief moment of screen time, making it that much more impactful.
Hermione on the other hand becomes too perfect and central to the films over time because she gets all the great lines and gets to stand up in all the great moments. She becomes less flawed than the character in the books, and that is a real disservice to her. In the books, Hermione has plenty of flaws, and that is what makes her interesting. Perfect characters are boring, and always trying to force Hermione into the center of the action is an error in the film adaptations.
There is no denying that the filmmaking craft that went into Harry Potter and the Prisoner of Azkaban is masterful. The movie is one of the best in fantasy cinema, but this one mistake in the adaptation process hurt the film, its characters and the series. Ron Weasley should have been more than comic relief in the films, and Hermione had plenty of her own moments in the books to be proud of, there was no reason to do both characters a disservice by giving one of Ron's best lines to Hermione.