For composer Hildur Guðnadóttir, the last year has been “beyond crazy”—a whirlwind that she still hasn’t fully been able to fathom. Earning her first Emmy for her work on HBO’s Chernobyl, Guðnadóttir hit on another water-cooler phenomenon with Todd Phillips’ Joker, tapping into the “psychological turbulence” spiraling within an increasingly furious outsider, with one of the year’s most unique scores.

A standalone origin story for one of DC’s most iconic villains, set in ’80s Gotham City, the thriller follows Arthur Fleck (Joaquin Phoenix) a mentally troubled man, who becomes increasingly disillusioned with the society that has rejected and abused him. Lashing out with a series of violent crimes, Fleck spirals ever downward, ultimately assuming a new identity, as the Joker.

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From her first read of the script, Guðnadóttir felt a strong connection to Arthur, centering her score on the cello, an instrument which had always been a prominent voice in her solo work. “The story is a journey of one man trying to figure out his past and where he comes from, so I thought it was really important to almost be able to get inside his head,” the composer explains of her process, and the thinking behind her Joker score. “It was really important to [examine] what it must have felt like to be the soul of Arthur Fleck.”

While arranging her score, Guðnadóttir thought about a man lost in a circular train of thought—a man who tried as hard as he could to fit in, and failed to do so, time and time again. “To me, it was really important that all of the music for Arthur is direct and completely without flourish—very simple, almost naïve. As you can hear in the score, there’s almost no harmonies,” the composer notes. “It’s almost like one chain of thought that’s carrying through, this kind of relentless simplicity.”

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Lingering behind the cello throughout Joker is a 90-piece symphony orchestra, which only rises to the surface of the score, as Arthur begins to connect with the anger inside of him. Toward the beginning of the film, the orchestra is hardly audible, hiding behind the cello, in the same way that Arthur has hidden from himself for most of his life, burying his anger, hurt and resentment behind a false laugh or smile. “As we go further into the story, and he starts to understand more, and his anger starts to come a bit more on the surface—especially towards the end of the film—the orchestra has become so loud that it kind of has eaten the cello,” Guðnadóttir says, “and the Joker has taken over the Arthur Fleck we saw in the beginning.”

For Guðnadóttir, the experience of crafting the Joker score was a visceral one—one that affected her physically, in a way that is difficult to describe. As she sat down with her cello and came across the notes that ended up being Arthur’s main theme, “it was almost like being struck by lightning in my chest,” she recalls. “I was like, there it is. That’s his voice; this is what he’s trying to say. It was the strongest physical click that I’ve had between a script and music.”

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On Phillips’ film, the composer found herself in the rare position of having cues played on set, and seeing the music she wrote directly influence performance. To Guðnadóttir, the magic of this experience manifested in a scene in a seedy bathroom, midway through the film. While it wasn’t initially scripted, this powerful moment came to represent the beginning of Arthur’s metamorphosis.

Watching this scene for the first time, Guðnadóttir was blown away to see Phoenix’s balletic dance, performed to her cue. “It was so magical to see that what Joaquin was doing was so, so similar to what I had experienced, physically. It was just mind-blowing to see how that communication could travel with so little speaking about it,” the composer shares. “It was one of the most beautiful collaborative moments I think I’ve had.”

For Guðnadóttir, what’s been beautiful with both Chernobyl and Joker is seeing her work register so strongly in popular culture, striking a nerve while opening up discussion about issues she considers important. “I’ve been working on film music for probably 17 years—but largely European productions, and productions that haven’t been very high profile. It’s truly wonderful when you get such a strong sense of people truly listening to what you’re wanting to say,” the composer tells Deadline. “I can feel people have such open ears and eyes towards these projects, and it’s truly gratifying when you feel that the work that you’re doing through art can have a big say in real life.”