"I actually look back on it now and think, 'F---ing hell, how did I do that?' " the actress admits

By Jacqueline Andriakos
Updated March 25, 2015 10:30 AM
Credit: Bjarne Jonasson/Self

Whether she’s sprinting on the set of Game of Thrones in a corseted gown or filming battle scenes beneath armor for Hunger Games, Natalie Dormer makes it seem effortless. Turns out, it’s a lot of hard work.

The actress, 33, opened up about her cardio secrets, interval training and green juice obsession in the April issue of Self.

“In Mockingjay – Part 2, I’m basically running around in heavy armor while carrying a semiautomatic rifle,” she said in the new cover story. “I don’t know how I would have kept up with [my costars] Liam Hemsworth or Sam Claflin if I hadn’t been in the best cardiovascular shape of my life.”

So how did the British star increase her endurance? Hint: without a personal trainer.

By day, Dormer took to running the hilly streets of Atlanta, where Mockingjay was filmed; by night, she squeezed in hour-long interval workouts. (And she still woke up every morning at 5 a.m. to 12-hour days filled with intense battle scenes.)

As if that schedule isn’t exhausting enough, Dormer topped it off by training for the London Marathon in April 2014. She signed up before learning her Mockingjay role would require her to report to set before dawn, according to the magazine.

“I actually look back on it now and think, ‘F—ing hell, how did I do that?’ ” she told Self, adding that she survived with a combination of midday naps and green juice.

“When you have early calls, you don’t necessarily want to eat first thing in the morning,” she said, calling her healthy juices a “good nutritional compromise.”

Dormer’s workout dedication also shaped her famed character of Margaery Tyrell – King Joffrey’s cunning young wife – in HBO’s Game of Thrones.

“Being strapped into a corset with this long flowing skirt immediately changes the way you look in the mirror,” she told the magazine. “My way into Margaery is actually quite physical, even though she’s not a physical character in the way that Cressida [from the Mockingjay movies] is,” she continued.

RELATED VIDEO: Brad Pitt or Michael Fassbender? Natalie Dormer Weighs In