Harry Potter and the Prisoner of Azkaban

The third Harry Potter film is where the series stops feeling like a school story. It also plays better the second time, because almost every odd little detail turns out to be evidence. First time through it is a 3.5. On a rewatch it is a 4, and the difference is entirely the film's own doing.

What Alfonso Cuarón changed

Alfonso Cuarón took over from Chris Columbus and rebuilt the look of the series in one go. Hogwarts stopped being lit like a television set and started being lit like a place, the students began wearing their own clothes, and the castle gained a clock tower and a hillside. A bus that should not fit between two lanes of traffic is stranger and funnier when the street around it looks real.

Aunt Marge inflating over the garden and the Knight Bus ride that follows do little for the central mystery.

Cut either and the mystery survives intact. They exist to tell you the wizarding world is bigger and odder than Chris Columbus let on, and at that job they both work. The Knight Bus is where the horror technique running under this film first becomes obvious: its conductor and driver are shot like something out of a different genre, and they land harder than the werewolf does later.

Albus Dumbledore, Remus Lupin and Sybill Trelawney

Albus Dumbledore (Michael Gambon) is the biggest change, and it is a writing change before it is anything else. He is arch now, and plainly enjoying himself, dismissing an executioner with thanks, announcing that he will be having a large brandy, whacking a patient's broken leg on his way past. The costume follows the script rather than the other way round: tie-dyed silk that floats behind him, instead of heavy ceremonial robes.

What that buys in charm it spends in gravitas. The script hands this headmaster jokes where the earlier films handed him authority, and the aura of a man the whole room defers to goes with them.

Sybill Trelawney (Emma Thompson) is the one worth the ticket. Emma Thompson sketched the look herself, from the idea of a woman who has not checked a mirror in years, and she is genuinely hard to recognise underneath it. Remus Lupin (David Thewlis) turns up looking like he has not slept in a week and proves to be the best teacher Harry Potter (Daniel Radcliffe) has had, and the one who gives him somewhere to put the sound of his mother screaming.

The Dementors and the Patronus

The Dementors are where the growing-up idea stops being a lighting change. They make Harry hear his mother die, which turns grief into something with weight and a temperature. Remus Lupin's answer is to teach him a spell strong enough to stand against them, and the Patronus lessons hand an old wound back to Harry as something he can use.