Of the scene in which Leonard Bernstein conducts the London Symphony Orchestra at the Ely Cathedral in 1976, Bradley Cooper said, "That scene I was so worried about because we did it live... I was recorded live. I had to conduct them. And I spent six years learning how to conduct six minutes and 21 seconds of music. I was able to get the raw take where I just watched Leonard Bernstein [conduct] at Ely Cathedral... And so I had that to study."
Steven Spielberg was considering directing the movie, but when Bradley Cooper showed him an early cut of A Star Is Born (2018), Spielberg walked over to Cooper after the "Shallow" scene and said to him, "You're directing Maestro." Spielberg ended up directing West Side Story (2021), featuring music by Leonard Bernstein.
Bradley Cooper admitted that one of the challenges in playing Leonard Bernstein was that so much footage of him conducting exists, and he couldn't possibly replicate his talent. Cooper therefore focused on a single performance, Mahler's second symphony, which he spent years learning how to conduct convincingly as Bernstein.
Bradley Cooper said his interest in Leonard Bernstein "started with Tom and Jerry and Bugs Bunny as a kid, watching them conduct, and asking Santa Claus for a baton when I was about 8 or so, and then, just being obsessed with conducting. There was a magical quality that you just do that, and sound comes out, you know, and that's where it began, and it was just this incredible feeling of being a maestro, really."
Carey Mulligan dropped out of her next project, Fingernails (2023), due to supplementary filming for this film. And she was 12 weeks pregnant at the time of filming.