As David Lowery, the director, was writing the fictional pop star Mother Mary for his new film of the same name, he spent a lot of time studying the last 25 years in music. He listened to Taylor Swift (whose Reputation concert film inspired the performances in the film), Lorde and FKA twigs, who appears on screen as a medium named Imogene. But as the film’s haunted love story between Mary (played by Anne Hathaway) and her former best friend and designer Sam Anselm (Michaela Coel) emerged, his listening habits shifted.

“The pop music fell away and other music started to enter that sphere,” he says in A24’s New York offices. He’s sitting beside twigs and Hathaway the day after the trio attended the film’s premiere in the city. “James Blake and Aldous Harding really captured the emotion that I was trying to type out between Sam and Mother Mary. They began to help me channel the feeling of the movie itself.”

The film is split into two contrasting modes: Hathaway and Coel largely enact a two-woman play within the confines of designer Sam’s atelier, where a broken and humanized Mary begs her former friend for a dress so good she could have a proper pop rebirth. The other part is Mary in god-like pop diva mode, commanding the stage with enough otherworldly magnetism to make clear that this star has created a cult-like, multi-generational fanbase. But Hathaway needed to unlock the broken, desperate, pleading humanity of Mother Mary first; she went into the movie largely blind of what the songs would sound like, save early demos of Burial and Holy Spirit, written by Charli xcx and Jack Antonoff.

“There were almost department heads for different aspects of the character,” she says. “I felt like her sound was pretty low on the totem pole, especially when we began.”

While the film itself is already divisive in early reviews, the music and performance footage has been acclaimed. Antonoff and xcx wrote the majority of the soundtrack, save My Mouth Is Lonely for You, a shimmeringly erotic offering from twigs that was left on the cutting room floor during her Eusexua sessions.

“I really love the lyrics, but I knew it wasn’t for me,” twigs tells me. She sent two songs to Lowery when he mentioned he needed more for the film; the other is more “ethereal” and used during a scene featuring the dress Mary requires from Sam. “As soon as David said he needed a song, I knew I wrote [My Mouth Is Lonely for You] for a reason.”

Even though twigs’ contribution was made separately from xcx and Antonoff’s work, the total soundtrack package paints a very specific and uniform portrait of a 21st-century pop star big enough to sell out large venues but weird enough to have both a cult following and avant-garde fashion sense. It’s not unlike what twigs and xcx have experienced of late; both are over a decade into their careers and have recently taken home first Grammys and headlined arenas for the first time. However, the implication of both the music and her characterization is that this fictional pop diva has a slightly glossier mass appeal. (Thankfully, the film doesn’t waste time breaking down the nuts and bolts of her commercial success and lets the performances and songs speak for themselves.)

“Mother Mary for me is the type of style that’s almost on the other side of the glass to what I am,” twigs says. “Even in my own industry, there’s a type of stardom I’ve watched through a window that she really embodies. Everything’s so together and so neat and so perfect and so huge and overwhelming.”

While Hathaway has a background in theater and won an Oscar for her performance in the musical Les Misérables, the recording process for Lowery’s film was a very new experience. She spent time with Antonoff to record Mother Mary: Greatest Hits, as the soundtrack is aptly named. She needed to not just explore her vocal range but also how production works. She even changed minor lyrics, having lived with the character long enough to understand what she would or wouldn’t say. Alongside Antonoff, she pushed for the type of sound layering she heard on Tori Amos’s Little Earthquakes, a huge reference for her as she dove into Mary.

“One of the things I realized is that lyrics are very important, but the feeling of the lyrics is the most important,” she explains. She cites the story of Earth, Wind & Fire recording September as an example, recalling how they didn’t quite understand the song’s lyrics or meaning but still found a way to transform it into one of the most instantly recognizable hits of all time. “The way you perform the sound of a word is just as, if not more, important than the word itself.”

There’s a specter looming over Mary and Sam, but the unspoken ghost in the movie is the implied fandom Mother Mary has attracted. Lowery felt it was unnecessary to make explicit; it’s clear through scenes backstage and the type of recovering physical shape she’s in when she surprises Sam that she’s pushed her body past the limit for these shows. It’s also clear through her own name and the ever-present halo she wears that her stardom means a lot to many millions of fictional fans waiting for her comeback.

“In the original script, there was a lot of explanation about who Mother Mary was as an artist, the depth of her fandom, what her songs had come to mean for people,” Hathaway says. The fandom is an abstraction, suggested through looks, passing headlines and, of course, the songs. But Hathaway does have a clear definition of who is drawn to Mother Mary; it’s not unlike how we see every other pop star functioning these days, comparing her to a “neon plasma” that could only be seen within the confines of a glass container who is struggling to not shatter the glass and hurt those who look up to her.

“Her fans are people who could feel safe around her. They could come to her for a feeling of ecstasy as whoever they were. Everybody was welcome,” she explains. “She loved them so much. They were people that she saw as vulnerable, people who needed a mother. She was twisting herself into this terrible state to avoid hurting them.”

• Mother Mary: Greatest Hits is out on 17 April. Mother Mary is out in US cinemas from 17 April, in the UK on 24 April and Australia on 14 May