Few adaptations have been pored over and scrutinized for adherence to cannon as intensely as the Harry Potter films. Every omission and deviation gets pounced upon immediately. Speculation ran rampant that each of the later, longer volumes would be split into two films to accommodate J.K. Rowling's sprawling storylines, until it was finally announced that the last book, Harry Potter and the Deathly Hollows, actually will be. In a few weeks, I'll take a look at the prospects for David Yates' Harry Potter and the Half-Blood Prince, due this November. This week, I want to look back at the sole Potter installment to date where the film not only did right by the book, but expanded it, improved it, brought it to life. And that would be Alfonso Cuarón's Harry Potter and the Prisoner of Azkaban.

I'm usually lavish in my praise for the film; I'm fond of saying that I like it better than Cuaron's purportedly more "serious" works like Children of Men and Y Tu Mamá También. What I don't often get a chance to mention is that I'm much less enamored of the novel on which it's based. Don't get me wrong -- Rowling's Azkaban is still Harry Potter, and as such it's fast, and funny, and filled with all sorts of wonderful, world-building detail. But when I read it, shortly after tearing through The Sorcerer's Stone and The Chamber of Secrets, I couldn't help but be a bit disappointed. It seemed a little contrived, I thought, and overdramatic; a little cheesy. The climax involved a lot of ALL-CAPS YELLING to signify big emotion, the whole thing feeling like it was about to turn into a wizard soap opera. And I remember rolling my eyes at the time-travel, which felt like a cheat despite being gracelessly telegraphed a dozen times.

So what went right? How did my least favorite Harry Potter novel become the crowning achievement of the franchise, book or film? It wasn't really the screenplay by Steve Kloves (the screenwriter of every movie in the series except Order of the Phoenix), which is solid and does most of the right things, but is also basically par for the course. Instead, Azkaban is elevated by a set of brilliant and inspired moves on the part of Alfonso Cuarón, proving that all the hand-wringing over who directs Harry Potter has been justified, after all.

Most crucially, Cuarón lets the characters – especially Harry, Ron and Hermione – spread their wings and function outside the confines of Rowling's plot. The pleasure is in the details, the little stuff: the way Ron grabs Harry's shoulder and turns him around when Draco Malfoy and the Slytherins start their taunts; the way Harry instinctively shields Hermione with his body when the dementors start circling in the film's frightening climax. The scene where Harry goes for a ride on the hippogriff moved me to tears, because it's not just a frivolous CGI frolic – it's Harry's momentary, joyful solace from the harsh reality that awaits him below. In Cuarón's hands, the characters behave like people, like teenagers, and like friends. Rowling is often able to accomplish this in her novels via the omniscient narrator, but that's hard to replicate on the screen if all you're doing is transplanting the book's storyline. Cuarón took the time to translate the characters' humanity into the language of cinema.

Along the same lines, the climactic confrontations in and around the Shrieking Shack don't threaten to become ludicrous. The same fierce anger is there, but now it's plausible, because throughout the movie Cuarón allowed Harry to get angry – to yell and rage and kick furniture – as everyday teenagers might, and not necessarily just as the plot required. (Even Rowling was never really able to pull this off.) It's also Daniel Radcliffe's finest moment as an actor; he does anger and dismay much better than the resolute heroism that the other films have mostly demanded.

Cuarón's second most impressive accomplishment is making Hogwarts feel like an actual physical place, with a determinate geography. He accomplishes this partly through his penchant for lush, beautiful long takes, a technique that lets screen spaces retain their geographic integrity much better than a barrage of cuts, and partly through simple paying attention. Chris Columbus' versions of Sorcerer's Stone and Chamber of Secrets made Hogwarts very picturesque and impressive, but it was not until Azkaban that I felt like I had a feel for Hogwarts as an entity (at least beyond the image I had from reading the books).

Aside from being valuable in its own right, this goes a long way toward making the elaborate time-traveling third act coherent, and even rousing. When the characters spend significant amounts of time stalking themselves around the Hogwarts grounds, doing their best not to be seen, it helps tremendously to have a sense of where they are in relation to each other (themselves?), where they've come from, and where they're going. What in the book seemed kind of like a poorly foreshadowed stunt becomes genuinely suspenseful, a highlight of the entire franchise. And it's all because of how rigorous Cuarón is, how careful, how precise.

The end result is this: Harry Potter and the Prisoner of Azkaban is the only one of the Potter films that breathes independently of the source material. It's not the most faithful adaptation, or the most "complete" one, but it's far and away the best. Cuarón understands the story, and he understands the characters – their personalities, their angst, and their pain. But most importantly, he understands movies. I like all the Harry Potter films – by and large, they all do an adequate job of putting Rowling's books on the screen, and the books are pretty darn good. But Azkaban is the only one that breaks free and reaches for greatness on its own.