"Don't reach down for your popcorn, because by the time you do, I could be off the screen, in Half-Blood Prince I am literally like, whoa, there she goes! But everyone was like that. In our first scene, Helena Bonham Carter and I are rolling past this blue screen, and we knock on the door and Tim Spall comes out.

"Tim pulls open the door and looks. That took about a day and a half of filming, and then it was, 'Thank you very much ladies and gentlemen, that's a wrap on Mr Spall,' and you go, that's it? You have people like Tim, that most producers are killing themselves to work with, and that's the fun of doing (Harry Potter]. Everyone is giving their all and they're all used to playing the lead parts. They're going, 'How shall I open the door? Do you want me to do it like this, or like this?' That sort of attention to detail, because, you know, they probably last did Hamlet. So David is surrounded by actors going, 'Right, I can give you this, or I can give you that, or I can give you . . .' and he's like, 'Please, please, just say your one line and [expletive] off.'" She laughs heartily. "He's extraordinary, because it's a huge project: the pressure!"

"In this film it's about protecting her son. The fact that you have children probably makes a bit of a difference, but not enormously, because you do spend a lot of time as an actor trying to imagine what the other person's feeling, anyway. As a woman you've played mothers before, and you don't have to be a mother to understand what feeling protective is all about."

She makes life at Hogwarts sound like a gala evening for Thespians Reunited: Alan Rickman, Jim Broadbent, Michael Gambon (with whom she made her television debut in 1993, in The Entertainer) and her hero, Tim Spall, required to do nothing more than hobble down a corridor and open a door. “You think, ‘What a waste’, but it’s ideal casting, because it doesn’t matter how good your Hamlet is if the soldier at the back’s not concentrating.”

Cinched and squeezed in sexy couture, she plays the mean beauty Narcissa Malfoy, a role she chose not because she wanted to pick up a wand for her kids, but because she was “blown away” by Potter director David Yates’s sex-trade drama, Sex Traffic. “I know,” she laughs. “Maybe an unusual reason.”