Dame Maggie Smith finds it hard not to burst into laughter when she is playing the part of Professor Minerva McGonagall in the Harry Potter movies.

She blames the films' scripts for giving her and her co-stars the giggles.

Maggie explains her dilemma: "It's hilarious. You feel such a berk.

"There's this wonderful first assistant and he'll be saying, 'Now Harry goes down among the dragons.' And you have to hold yourself because if you lose it for a second you're sunk."