On one side, Maggie Smith is backed by a sea of kids in uniform. On the other is Voldemort's motley crew.

The cast - Emma Watson, Bonnie Wright, Matt Lewis, Evanna Lynch, Warwick Davis - file in to join the 35 adult extras, 102 young people and 20 stand-ins already on set.

One of the stand-ins mimes an absent Ralph Fiennes's lines as Voldemort, goading the side of good. "Harry Potter is no hero," he bellows. "From this day forth you put your faith in me!"

Voldemort calls for Draco Malfoy (played by Tom Felton) to cross over to his side.

Not needed on set, Harry Potter himself ducks out of the rain and sidles up to Hit. Turning his back on the action, Daniel Radcliffe grins and says: "I know all this. I don't need to see it again."

"This scene is cursed or something," [Radcliffe] says, shaking his head. "We've been filming this scene now over a period of probably eight months on and off, coming back and back and back to it in all states of weather - snow, rain, sun.

"Every time we come to it there's always a hitch somewhere along the line. But it will be an amazing scene."

Director David Yates calls cast and crew back into action. Radcliffe lowers his voice and whispers, "Let's all watch."

"This particular bit, Voldemort gives a call to arms and offers a bit of an amnesty. He says, 'Anybody who wants to swap sides, now is your time'. So it's a call to everybody on that side of the court," he gestures off to the left, "who's fighting for Hogwarts, to join him."

By this time, Yates has moved filming to the second part of the scene - here Lewis, as nerdy Neville Longbottom, reveals unknown bravery, stepping forward to stand up to Voldemort and defend Harry's honour. He sparks a chain of events that lead to what Radcliffe describes as "a 20-minute battle".

"It's the fact they keep it in that bag that makes it so much worse. I saw it and I was like, 'Oh, wow, I look quite good'. It is slightly odd, I s'pose, but it's one of those things you become more and more accustomed to seeing, and less surprised by."

"Pretty much as I imagined from the script, certainly," he says, more matter-of-fact than wistful. "Once the script is out I don't really go back to the books too much. But yeah ... it's as epic and as frightening and dark as it needs to be."