One of the most perplexing and irritating aspects of Hollywood is the way it continues to churn out samey, mediocre fare based purely on the financial success of previous instalments. Having endured the first two Harry Potter films, and seen them gross more than $800m each, we might have expected the series to plod along ad nauseum, with every year or so bringing us another disappointing and inconsequential adventure for the little wizard. Fortunately for all concerned, that didn't happen. With Chris Columbus leaving the director's chair to spend more time with his family, the time was ripe for a fresh pair of eyes to come in and give the franchise the kick up the arse that it badly needed. With The Lord of the Rings trilogy now completed, garnering rave reviews and 11 Oscars, this would be the film that would need to justify going on. While Harry Potter and the Prisoner of Azkaban never quite comes close to Peter Jackson's brilliance, it is far superior to anything that came before in the series and set the bar very high for all that came after. Alfonso Cuarón may now be a name toasted among film fans, following his gripping work on Children of Men and the groundbreaking visuals in Gravity. But he was by no means a shoe-in to direct. Guillermo del Toro said that he wasn't interested because it was too "bright and happy and full of light", while Marc Forster turned it down because of his work with child actors on Finding Neverland. In the end it came down to a three-way choice between Cuarón, Thelma and Louise screenwriter Callie Khouri, and Kenneth Branagh, who had appeared as Gilderoy Lockhart in the previous instalment, Harry Potter and the Chamber of Secrets. The box office failure of Love's Labour's Lost probably deprived us of the heady excitement that would have been a Branagh-helmed Potter film. But there can be no doubt that Cuarón was a great choice, for one simple reason: he understands how fantasy storytelling works, particularly how any fantasy film must be underpinned by a sense of wonderment. Columbus' pathological desire to faithfully replicate the books on screen had robbed J. K. Rowling's source material of all its sense of wonder and amazement. Cuarón restores the balance, giving us a darrker plot with more developed characters and never losing sight of the magic in amongst the spells. Put simply, all the details of the world of Harry Potter, and the story in which he finds himself, are more fully and strikingly realised in Prisoner of Azkaban. Cuarón gets a lot of the big stuff right early on, whether it's the light-hearted comedy of the Knight Bus or the chilling design of the Dementors. But he also puts in a lot of effort to make all the little details fit, particularly during the scenes at The Leaky Cauldron. Whether it's the self-stirring coffee, the angular corridors, the carnivorous books or the silently screaming wanted posters, you always feel that what you are seeing is magic, rather than constantly being told that something is magic. This magical feeling is reinforced by the film's colour palette. In my review of The Philosopher's Stone, I likened its visual style to sanitised adaptations of Charles Dickens which are common in America; as a general rule, I argued, British adaptations of Great Expectations, Oliver Twist and the like are more content to leave all the edges on. This film is shot by Michael Seresin, Alan Parker's cinematographer-of-choice who would later team up with Cuarón again on Gravity. Cuarón clearly understands the darker, more distinctly British roots of the series and the genres upon which it draws, and allows Seresin to work his murky magic with piercing blues and encircling blacks, making it look like a horror film for children. From a narrative point of view, Prisoner of Azkaban benefits greatly from the direct absence of Voldemort. On the one hand, the fact that he doesn't appear physically or personally in the story spares the film from becoming too repetitive or formulaic: it isn't simply retreading old ground and prolonging the inevitable. This is the trap into which the later Potter films often fell: we always knew that things would end with a showdown between Harry and Voldemort, and the more the films went on without it happening, the more irritating the tease became. On the other hand, the Dark Lord's influence remains a background threat, allowing Cuarón to build a more intimidating atmosphere. In the first two films, Harry was constantly on the lookout for the man himself: we always knew who the bad guy was, even if we didn't know what form he would take. With Voldemort having to prey on Harry indirectly, he is forced to choose his friends more carefully, and learns over the course of the film just how deceptive appearances can be. By putting Harry on more insecure footing, the film manages to epitomise his adolescent confusion about the world, and the conflict he faces in fulfilling a role which he didn't choose. The first two films often painted Harry as a goody-two-shoes who had perfect parents, but here we get the first hints of things being a little more nuanced than that. When Harry watches the Dementors kill him and Sirius, expecting his dad to come in and cast the patronus charm, he begins to realise that he cannot rely on their reputation if he is to grow as a wizard. While the next film in the series goes into this in more detail, threatening to tear apart the three central friendships, Prisoner of Azkaban does a good job of setting this up. This brings us on naturally to the time travel element of the story. Anyone familiar with Back to the Future Part II, Twelve Monkeys or Timecrimes will instantly recognise the territory we are in, with the characters constantly trying to avoid running into themselves, and events being repeated from different perspectives. Coming as it does towards the end of the film, our natural instinct is to conclude that it's surplus to requirement, one twist too many. But if we allow it to play out and watch as all the pieces fit together, we discover that it ties into the central themes of trust and reputation quite nicely. Besides, if we can accept the existence of horcruxes, sorting hats and hippogriffs, time travel isn't much of a stretch. Despite all its darkness and intimidation, Prisoner of Azkaban also manages to be pretty funny. The scene where Potter beats up Malfoy and his thugs near the Shrieking Shack is a classic piece of British slapstick, with Tom Felton showing excellent timing through his many pratfalls. The interplay between Snape and Sirius is like a double-act in itself, with Sirius' manic energy coming up against Snape's stonewalling seriousness. Throw in the first scene with the boggart and most of the opening section, and you have a film which achieves a fine balance of light and shade. With the young cast continuing to improve and growing into their roles, Prisoner of Azkaban also has a couple of aces up its sleeve in the adult cast. David Thewlis was an ideal choice for Lupin: he revels in his lanky physicality and whistful delivery, in stark contrast to his more tight-laced performance in Gangster No. 1. Gary Oldman's performance is equally remarkable, being much more operatic in scale but still retaining a gentle, tortured humanity. Oldman resists the urge to simply ham it up and play mad, imbuing Sirius with a tormented quality which makes him compelling. In spite of all its successes, there are still a couple of problems with Prisoner of Azkaban. Despite the best efforts of Cuarón and screenwriter Steve Kloves, elements of the plot are still repetitive or predictable. After three films, it's fair to presume that whoever holds the post of Defence Against the Dark Arts is going to be a villain in some form, and it is equally certain that Dumbledore will pop up at key moments to misdirect, drop hints or mischievously intervene. These tropes aren't enough to derail the action at any given point, but they are pinches of salt to be taken whenever anyone describes this film as ground-breaking. Additionally, the film still feels longer than it should. This was the shortest of the three films at the time, at 142 minutes, and it remained the shortest until Order of the Phoenix three years later. But while the pacing is better than Columbus' offerings, it still feels like there is an awful lot of plot being squeezed in when the sensible thing would be to cut more stuff out. Cuarón keeps the fans at arms' length, deciding what he wants to shoot and leave in, but another couple of swift edits to get it down to two hours wouldn't have hurt. Harry Potter and the Prisoner of Azkaban is a massive improvement on the first two films, justifying the continuation of the series and helping to explain its appeal to those out of the loop. While it's still too long and well-worn in places, it largely makes up for this with strong performances, interesting themes, much better pacing and a visual style that combines horror and fantasy to leave us really impressed. Whether it's the best Harry Potter film remains a matter of debate, but there is no debate that the later films owe their existence to its success.

Cuarón is definitely a better director for this adaptation than Columbus, since this is a darker, more character-centered story compared to the previous two, and the result is more condensed and not so faithful, which works for the best as it leaves out some of the book's few problems.