For Maisie—who was monitored by a team of medical professionals to supervise her drastic weight loss—explained that this role was far more taxing than battling monarchs as Arya Stark on GoT.

As she put it, the part “took over what I’m eating and how I’m moving and sleeping and thinking.”

Maisie’s transformation wasn’t just physical, either.

“There was a lot of feeling restricted, almost like sleep paralysis, dreams of being trapped and attacked, and horrible visions of men in uniform,” she shared. “Every day of filming was a reminder that we were portraying a story of the horrors that humans are capable of inflicting on one another, but also the magic and the hope and the love.”

Despite the intense process, Maisie felt it was her duty to pay homage to Catherine’s legacy.

“It was hard work, but it was such an honor to do this part, and it became all-consuming,” the HBO star added. “This was a role that required an extraordinary amount of research and care—and it really hit me, bringing that wartime period back to the streets of Paris; like I could acknowledge the weight of it in a way that I hadn’t before.”

Maisie isn’t the only star to alter her appearance to get into character. Keep reading to see all of the celebrities who gained or lost weight for roles.