Jordan Peele’s horror film Us has raked in over $174 million worldwide since its opening, and critics are describing it as “fiercely scary” and “truly terrifying”—and a large part of the credit goes to Lupita Nyong’o’s uniquely spine-chilling performance. The Academy Award-winning actress is proving to be quite versatile with roles ranging from the period drama 12 Years a Slave to Marvel’s Black Panther to now portraying dual characters in Us.

The 36-year-old actress faced several challenges while working on the film. First of all, she plays two characters who are vastly different from one another—one being the hero, Adelaide Wilson, a woman who suffered some sort of childhood trauma, and the other one being the doppelganger of Adelaide called Red who belongs to the vengeful army called “the Tethered.”

As far as the shooting schedule was concerned, Nyong’o only had to embody one of her two characters on any given shoot day. This arrangement helped her to maintain focus on both characters throughout the course of filming. One of the producers, Ian Cooper, told Variety“[Lupita] would get into that [Red] character and inhabit it all day, and I learned the hard way to not make small talk with her on the Red days.” As many scenes required both of her characters interacting with one another, Nyong’o was challenged to act without a scene partner. It was difficult because “the person” on the receiving end of the dialogue was just an eye line or a green cross on the wall.

The Mexican-born Kenyan actress loves the idea of giving people the creeps. But she admits the role did activate some of the dark recesses of her mind. In an interview with Ellen Degeneres, she said, “It was hard work. Because [the two characters are] very different, they are diametrically opposed to each other. But they are also connected. So I had to go to some dark places within myself to find the doppelganger, Red, do a crazy voice—that was a lot of fun to create.”

The script indicated that Red hadn’t used her voice in a long time. So Nyong’o found inspiration in a vocal quality based on an actual condition. “Well, I was inspired by the condition spasmodic dysphonia which is a condition that comes about from a trauma—sometimes emotional, sometimes physical—and it creates this spasming in your vocal cords that leads to an irregular flow of air. So I studied that,” the Yale graduate says. “I worked with an ear, nose and throat doctor, a vocal therapist, and my dialect coach to try to make sure I could do it and do it safely—because I had two roles to play and I couldn’t afford to damage my voice.”

She had to experiment in the beginning to see what worked best for her because she ended up with a sore throat at first. She also learned to warm up and cool down her voice on her Red days.

Indeed, her creepy voice and shocking laugh, in particular, are freaking out audiences. Cooper recalled when Lupita debuted her monster voice on set, saying, “The first time we heard the voice, we were shooting … and you could hear a pin drop. Crew members were scared hearing that!”

In addition to the disturbing voice, Red’s movements were physically demanding as well. Nyong’o worked on her two counterparts’ uniquely different movements for four months before the shoot began. Adelaide comes from a background in dance, so the glamorous and graceful Nyong’o trained for ballet three hours per day in order to carry herself with the specificity of a dancer. Then the challenge became how to make an elegant dancer’s movements appear to be very disturbing. Red was initially described to Lupita as a “queen” and a “cockroach.” In turn, she created a physical style paying tribute to cockroaches around the world—sometimes being very still and other times scurrying off quickly.

With all the challenges the two roles presented, Nyong’o says she took a lot of naps between takes “just trying to catch up with myself.” Now she can sit back and enjoy all the great reviews.