When Ezra Miller signed on to star as The Flash in the DC Extended Universe, he did so under the assumption that there would be a lot of pretend running. That's fair, given how many technological advancements there have been in the filmmaking industry. Even the Flash TV show uses a CG double to film the superhero's running sequences. Little did Miller know, however, that he'd be strapped to a 25-foot treadmill, actually running, to portray the Fastest Man Alive.
"Sometimes they make me run, which I was outraged!" Miller told Fox 5 DC during a press day for Fantastic Beasts and Where to Find Them. "I thought, 'You can't see him running, so this is going to be easy!' No, they were like, 'Ezra, we need you to run.'"
During production on Justice League, director Zack Snyder had Miller strapped onto a fancy treadmill to film these super-speed sequences.
"[It's] 25 feet long, goes up to 45 miles per hour, and it's bouncy like a trampoline — pretty much a dream come true," Miller said.
Finding a way to make The Flash's super speed look exciting and dynamic must have been a challenge for Snyder, especially with the blue lightening of the Speed Force in play. The first teaser trailer for Justice League gave us our first glimpse of Barry Allen in action when Bruce Wayne threw a Batarang at the speedster. Time essentially stopped (well, technically speaking, it just moved very, very slowly) while Barry was able to move freely, which was reminiscent of Quicksilver's scene-stealing sequences in the X-Men franchise. According to Miller, a lot of the Flash's scenes will play around with the idea of perception vs. reality.
The Flash's use of the Speed Force also inspired a lot of nerdy conversations in the realm of "How much does Barry Allen need to eat to break the sound barrier?"
"We constantly get to have the geekiest conversations about physics," Miller added. "We'd be like, 'He'd be so sweaty because he's running so much. But wait! He wouldn't be sweaty at all because all the sweat would evaporate the second it formed.'"
Just don't forget to load up on carbohydrates, Flash.