Alfonso Cuaron said the Prisoner of Azkaban was a much older book than the first two, when the kids have to deal with the problems of growing up.

"Harry goes through a journey where he realises that demons aren't just things that go bump in the night," he said.

"But also can be painful emotions, worries about family, friends, the future. That's a classic teen issue."

More modern

Alfonso added that Harry, Ron and Hermione have to figure out the forces of darkness while also trying to work out what it's like being a teenager.