When Renée Zellweger first saw herself in full Judy Garland garb—prosthetic nose, colored contacts, wig, and dramatic makeup—she couldn’t help but gasp. “I went, ‘Wow!’ Jeez, Judy’s features and her defining physical characteristics were instantly recognizable. It felt like it wasn’t me underneath the wig and makeup,” Zellweger said at Thursday’s Los Angeles premiere of Judy, the biopic in which she plays Garland in the last year of her life. “Every day it was denial. I couldn’t believe it was me.”

Set in the winter of 1968, a few months ahead of Garland’s death from an accidental drug overdose at the age of 47, Judy finds Garland in financial debt and deteriorating health. She accepts a singing job at London’s Talk of the Town, a fashionable cabaret club, in an attempt to earn money to provide for her young children. The film also flashes back to Garland’s childhood at MGM, where studio mogul Louis B. Mayer gave her pills to keep her moving through a grueling production schedule. “It wasn’t until I did this film that I properly came to appreciate how extraordinary she was by learning about her circumstances she had to grapple with and how she had to navigate through insurmountable difficulties,” Zellweger said. “She was not allowed to be a human being. There was no room on her schedule for her to think and be sane. Basically she was exploited. It empathizes [with] how truly extraordinary she was and what she was able to achieve in spite of that.”

Zellweger spent one year training with a vocal coach, Eric Vetro, before production began. She then rehearsed for four months with the film’s musical director, Matt Dunkley, to master her vocals. Even after starring in the movie musical Chicago, Zellweger found it “intimidating” to train to play Garland. “I couldn’t sing any of the songs initially. I didn’t have the strength to do it. Singing in the car was where it first started. I finally found a good use for L.A. traffic,” she said with a laugh. “Judy was riding shotgun for about a whole year with me.”

Each of Zellweger’s six musical numbers, including “By Myself“ and “Over the Rainbow,” was performed live on set in front of an audience to the sound of a band playing in her earpiece, while the cameras rolled in one take. “That was director Rupert Goold’s idea,” Zellweger said. “He decided to do everything live because he wanted to capture the shared experience that a performer has with an audience. I’ll forgive Rupert one day for doing that to me. It wasn’t easy.”

“Those were big, big songs that Renée sings, and there was no room for error,” Goold said on the red carpet. “It was live and it happened in real time. We stayed on Renée and never cut away to show the authenticity of her performance. She had people really crying in the audience.”

“Over the Rainbow,” as Garland’s most famous song, was a particular challenge. “I’m not going to lie, there was some personal pressure to get it right,” Zellweger said. “Because that beautiful song speaks to us all in a different way, and to me, it’s about hope. We have nostalgic feelings from childhood attached to that song, but in Judy’s life, it’s something different. She weathered so many insurmountable challenges in her life, and it’s about her maintaining hope. In spite of all her difficulties, she still carried on. It’s deeply moving.”

Once Zellweger nailed the cadence and tone of Garland’s voice, she relied on hair and makeup artist Jeremy Woodhead to meet the physical demands of the role. Zellweger was fitted with a prosthetic nose, brown-colored contact lenses, and pixie-cut wigs, in a process that took about two hours each day. In addition to her altered appearance, Zellweger completely changed the way she carried herself to mimic Garland’s slouchy posture, which was caused by a curvature of the spine. She also studied her mannerisms and copied her twitchy body movements during onstage performances.

“Playing Judy Garland was the most challenging and terrifying role I’ve done,“ Zellweger said. “At times, if I could have run away, I would have. But there is so much affection for her. What I found so compelling about this particular project is that it contextualizes that she didn’t make those choices for herself. I hope people will realize this and understand how truly spectacular she was.”

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