In this video, you're going to learn 15 behind the scenes facts about Harry Potter and the Prisoner of Azkaban.

The third film in the Harry Potter franchise was going to be very dark in comparison to the first two films.

And the series' new director–Alfonso Cuarón–wanted to prep the three young leads by having them each write an essay about how their characters had changed over the past two films.

When it was time to turn in their papers, Daniel Radcliffe had written a one page summary, Emma Watson wrote a 16 page research paper, and Rupert Grint didn't write anything at all.

However, instead of being disappointed, Alfonso was happy that each of their responses to the assignment showed how well they understood their characters' behavior.

Some other actors who had homework to do were: Gary Oldman, who had only read the first book in the series,

and David Thewlis and Michael Gambon who hadn't read any of the books before being cast in their respective roles.

However, all three actors committed themselves to read the third book before filming began.

"Welcome, welcome to another year at Hogwarts!"

Before Michael Gambon was cast as Dumbledore, a few well-known actors were considered for the role after Richard Harris passed away, and one of them was Sir Ian McKellen.

However, the problem was that before dying, Richard Harris said McKellen was a "passionless" actor.

"You shall not pass!"

So ultimately, the Lord of the Rings star turned down the role of Dumbledore because he felt it would've been inappropriate to replace someone who didn't approve of his work.

To help the young teenage actors begin to develop more unique personalities, Alfonso Cuarón gave the actors permission to wear their uniforms however they wanted.

In addition to that, he also wanted Harry, Ron, and Hermione to wear regular clothes more often than their Hogwarts uniforms,

to allow them to show more of their characters' personalities throughout the movie.

Harry Melling who played Dudley Dursley was almost recast because of all the weight he had lost between the second and third film.

But instead of casting someone else in his role,

the filmmakers decided to have him wear fat suits to maintain his pudgy appearance throughout the rest of the films in the series.

One of the new characters that brought a darker tone to the story were the ghoulish looking Dementors,

who were inspired by creatures J.K. Rowling saw in her nightmares while she had depression before her success with the Harry Potter novels.

To bring the Dementors to the big screen, Alfonso Cuarón was nervous about using CG at first, so he initially tried using practical effects with water tanks.

However, their tests weren't producing the results he wanted so he decided to have the Dementors be CG instead,

but they used the footage from the water tanks as a reference to help design the creatures' movement with the computers.

Even though the story's tone was darker, the cast and crew still had some fun to keep things lighthearted on the set.

For example, during the sleeping bag scene, Daniel Radliffe had asked to be placed next to a girl he liked,

but what he didn't know is that Michael Gambon put a remote-controlled whoopee cushion inside it

So the filmmakers could embarrass him while he was trying to stay in character for the shots.

The Prisoner of Azkaban was the first time the series was filmed on location and they used Scotland to serve as the Hogwarts castle grounds,

but filming there came with the challenge of dealing with the rainy weather conditions.

They decided to film in May since they believed it would be the least rainy month of the year;

however, when they got there, it rained every day for 28 days straight,

and the ground became so wet that they had to fly in a helicopter to dump gravel on the ground to keep it from washing out from underneath them.

Robbie Coltrain's costume was very heavy weighing more than 60lbs, and filming the forest scenes in his thick, heavy costume was very hot with temperatures rising up to 100 degrees outside.

So he was given a special vest to wear with plastic tubing that ran ice water through it to keep him cool while filming.

To film the Hippogriff scenes, they used a combination of practical and CG visual effects.

To help the actors' performance on the set, they used an animatronic that took four months to build,

and was so convincing that some of crew had to keep reminding themselves it wasn't a real animal.

To make Harry's flight with Buckbeak look more convincing, they placed Daniel Radcliffe on a motion rig, and then superimposed his performance on top of a CG Buckbeak.

And then to get the exterior shot of Hogwarts castle, they built a small scale model and then added all the components together during post-production.

The Knight Bus was actually a real vehicle that they built by cutting a double decker bus in two parts and then installing a middle section between them.

The Knight Bus was also built to travel twice the speed of a real London bus.

And to get the shots of Harry being tossed around inside the bus, they built a hydraulically controlled motion base in a sound stage to film the bus interior.

For Professor Lupin's transformation, the make up process was very long and tedious and took 6 hours to get David Thewlis ready for his scenes.

Even some of the animals went through a transformation for their roles, such as the cat they used for Crookshanks.

They put jelly and eye shadow around his eyes to make them look sad and angry,

and he went into hair and makeup every day where they took the loose hairs of his undercoat, rolled them into a ball, and clipped them to his fur to make him look mangy.

Is The Prisoner of Azkaban your favorite film in the series?

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