Alan Rickman reveals the secrets of Harry Potter

Emma Watson, Rupert Grint, Alan Rickman and Daniel Radcliffe in Harry Potter and the Prisoner of Azkaban.

The Harry Potter books are full of magic and spells. And it seems that a little trickery also had to be used on set when the books were being turned into films.

Daniel Radcliffe, Emma Watson and Rupert Grint were in fact played by "very small adults" wearing wigs for parts of Harry Potter and the Philosopher's Stone, the actor Alan Rickman has revealed.

The actor, who played Snape in the film franchise, said child labour laws had placed heavy restrictions on the filming of the early movies, leaving the young actors compelled to leave set to rest and do their school work.

In order to save time, Rickman said, any shots of Harry, Hermione or Ron in conversation with adult actors were filmed in part with the help of careful disguises.

Daniel Radcliffe, Emma Watso and Rupert Grint in Harry Potter and the Philosopher's Stone.

Calling the early days on set an "organisational nightmare", he said: "In the first film, if anybody ever wants to look, of course you've got the problem of kids who can only work a certain number of hours.

"Sometimes there were 300 children on set, and at certain points they all had to go off and do some schoolwork." When he was in scenes with Radcliffe, Watson and Grint, he said, he would be present while the crew filmed close-up shots of the children, then aged 11 and 12.

But when it was his turn to speak, "in would come the very small adult actors aged 33 with a wig on their heads".

Daniel Radcliffe in Harry Potter and the Philosopher's Stone.

At the time, child actors of school age were permitted to work for four hours each day.

Current legislation means children over the age of nine are allowed to spend five hours in rehearsal or performance, with a break in the middle.

Speaking at Bafta, as part of the Life In Pictures series, Rickman also told how JK Rowling, the author, kept the secrets of Harry Potter so close to her chest that the cast found out what was happening to their characters when they picked up copies of each book on publication day.

He also revealed how he once donned a disguise to join a family friend at a bookshop for the book release at midnight.

Stunned at the length of the queue, he eventually swept to the front of it and whispered who he really was to staff, who gave him the book immediately.