Harry Potter costars Evanna Lynch and Matthew Lewis keep close together as they launch 1,000 black balloons into the air at Millennium Bridge in London on Friday afternoon (September 25).

Each balloon that was launched has a ticket with a code that entitles the holder to enter a prize draw. The winner will appear in the final film as a moving portrait on the walls of Hogwarts school.

Matthew shared with BBC News, “The winner will be able to see all the stuff behind the scenes. People always assume the bigger sets are all computer-generated, but a lot of them are actually there.”

Evanna and Matthew are currently filming the last two parts of the Harry Potter series. Evanna revealed revealed that filming on the climactic battle for Hogwarts hasn’t begun. “The big stuff is still to come. I like that because if it’s all at the start it’s a bit of an anti-climax, but they’re keeping that bit until the end,” she said.