Daniel Radcliffe and Emma Watson decided to spice up filming on the set of HARRY POTTER AND THE DEATHLY HALLOWS by challenging each other to a race.
The pair, along with their co-star Rupert Grint, was instructed by director David Yates to "run like maniacs" for a series of frantic scenes, and the young stars relished the challenge.
But Grint struggled to keep up with his pals - as they were secretly competing with each other.
Yates tells ABC News, "They run like maniacs and it's because they are trying to out-compete each other. Dan wants to run faster than Emma. Emma wants to run faster than Dan, and Rupert just wants to be anywhere to try to keep up with the pair of them."
Grint adds, "I didn't realise it was going to be that fast actually, I was just keeping up with Dan and Emma."
The first installment of the two-part movie will be released later this year (10), while part two hits cinemas in 2011.