The Oscar winner gained 12 pounds of muscle to become the famousTomb Raidercharacter, working with trainer Magnus Lygdback to transform her body three months before she started filming the upcoming movie.
"On this job, it was the first time I was expected to do as much muscle and weight training as I've done," Vikander tells ET. "I've really felt it very empowering, and I still feel like I have a physique."
"I already wanted her to be a strong, physical girl," she adds. "This film is about her becoming the action hero we know her to be."
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Action hero bootcamp was no easy feat. ET's Nischelle Turner met up with Lygdback to get a taste of the Tomb Raider workout, where he put her through the same moves as Vikander.
"I mean, it takes a lot of work and dedication. And I would lie if I said it's a walk in the park," Lygdback says. "It's hard!"
For the trainer and his slew of celebrity clients (including Katy Perry and Alexander Skarsgard), it's all about quality over quantity. "People think they need to spend hours in the gym. With my method, that's not what we're doing," he explains. "Never more than one hour a day and not even seven days a week... you might do a little cardio in the morning and working out in the afternoon while cutting, but that's it."
And according to Lygdback, the hardest part for Vikander was taking a day off. "She's so dedicated and hardworking, so the hardest part for her was to take a day off and relax," he reveals.
It seems Vikander shares a few qualities with her character in Tomb Raider, which tells the origin story of the Lara Croft that Angelina Jolie portrayed in 2001. With Lygdback's help, Vikander packed a whole lot of muscle into her 5'5", 117-pound body.
"We didn't have a specific number when we stared. It's all about how you feel and how you look. It's about building the character," he says. "I think we really hit the mark. I'm happy with the way she moved, the way she looked and I think she made [the character] her own."