The "Suicide Squad" actress portrays Queen Elizabeth I opposite Saoirse Ronan as her cousin Mary Stuart in the historical drama, which focuses on the Scot's attempts to overthrow the English monarch and claim her throne.
Margot completely transformed her appearance to play the royal redhead onscreen, but she wasn't keen on having her eyebrows drastically thinned out for the role - and neither were the other ladies in the cast.
"We had to all have our eyebrows plucked to within an inch of their lives!" she bemoaned on breakfast show "Good Morning America". "Different fashion senses back then - it wasn't like the thick 2018 brow that you want, so we all lined up as... our hair and make-up designer was like, 'Nope, they're going!' We were all like, '(But) it took me so long to grow them!'."
"It was a bonding experience!" Margot chuckled.
The Australian actress almost missed out on the experience altogether as she had initially been reluctant to take on the part of Elizabeth I.
"Queen Elizabeth I has been portrayed by some of the most iconic actresses in the world, from Katharine Hepburn (in 1936's Mary of Scotland) to Cate Blanchett (in 1998's Elizabeth), and that was scary," Margot admitted. "But also she's a very iconic historical figure, and I just didn't see what I could have in common with the Queen!".
Director Josie Rourke eventually convinced Margot she would do a great job, and once she accepted the gig, she looked forward to working with the "incredible" Saoirse - although they only shared one pivotal scene together, a fictional meeting between their feuding characters.
"It's a great scene and it was worth it," Margot insisted. "It was a 12-page scene; you don't often get those."
"They kept us apart throughout shooting so we never saw each other in character... until that moment when the camera's rolling on our close ups...," she explained. "It was my last day and Saoirse's first day (of filming), and it was the only time we were on set together, and it was just wild."
"After the movie, we both got nominated (for other roles) and so we were kinda seeing each other every day at different events," Margot shared. "We spent more time together doing press throughout other movies than we did doing the film together!".