"I'm the oldest person in the cast and I have the most dancing, which didn't make sense to me," the 71-year-old Streep said in a Zoom press conference.
Streep plays Dee Dee Allen, the star of a Broadway bomb. She and her co-stars Angie Dickinson (Kidman), Barry Glickman (James Corden) and Trent Oliver (Andrew Rannells) travel to Cincinnati, hoping to rehabilitate their images from their Broadway failure.
Their plan is to rally behind a gay teenager, Ellen (Jo Ellen Pellman), whose school forbids her to go to the prom with her girlfriend, Alyssa (Ariana DeBose).
The Prom debuted on stage in Atlanta in 2016 and made it to Broadway in 2018. When Streep went to see the show, the Dee Dee character on stage did not have as much choreography as she would in the film.
"All hell broke loose when I got to Los Angeles and they laid out for me what it was," Streep said. "I got in shape. It's a lot of stamina. Man, it was hard work, but it was really, really fun."
Streep and Kidman had experience in musicals; Streep in the Mamma Mia movies and Kidman in Moulin Rouge! To prepare, director Ryan Murphy set up six weeks of dance rehearsals for the cast on the Paramount studio lot in Hollywood.
"When you're making a musical, [rehearsal] is where the cast bonds," Murphy said. "They kind of become like a Broadway troupe. Everybody cheered each other on."
The signature number for Angie is "Zazz," a jazzy song in the style of Bob Fosse, paying homage to Chicago and Cabaret. Angie is demonstrating the inspirational story of an understudy who had to fill in for a sick cast member as Roxie Hart in Chicago.
"It was terrifying because the Fosse dancing is so specific," Kidman said. "I had this amazing group of dancers who just trained me and trained me with the patience of saints."
Although the dances presented physical challenges, Streep said the benefits far outweighed them. Despite her complaints about aging and physical strain, Streep hopes the dance numbers she learned for The Prom make viewers happy.
"Whatever your particular sadness is, it's irresistible in movie musicals when people start to dance," Streep said.
In The Prom, the joyous music has a hefty task of uplifting a very delicate story. In addition to being forbidden from the prom, Emma faces intolerance every day from her classmates and community. Alyssa was planning to come out at the prom when she arrived with Emma, so their secret relationship adds stress to Emma's situation.
It was Emma's struggle that made Murphy want to adapt The Prom, the director said. Murphy created the musical drama Glee and the dance musical Pose. Murphy said he attended the show in 2019 as a fan and discovered he related to Emma.
"Halfway through the musical, I realized that was my experience," Murphy said. "I'm from Indiana. I was not allowed to go to my prom."
Murphy agreed with Streep that the show's uplifting music helped make its serious themes resonate.
"It had so much joy and optimism and it was about something," Murphy said. "Yet, it was also just fun."
At one low point for Emma, Angie and Emma also share an intimate scene in which Angie confesses her insecurities about being a chorus girl. This leads into the "Zazz" number, intended to cheer Emma and show her she's not alone.
"I just love that there's this older woman helping this younger person through by just being there for her," Kidman said.
Emma also brings up unresolved issues for Barry, whose parents rejected him when he came out as a boy, and the publicity he generates with Emma facilitates a reconnection with his mother (Tracey Ullman).
"For Barry, as soon as he sees Emma, he sees himself in her completely," Corden said. "I think part of him wishes he had somebody like him around in those times."
The Prom premieres Friday on Netflix.