Bradley Cooper spent six years studying Leonard Bernstein to play him and direct Maestro, in theaters Wednesday.

In 1973, Bernstein conducted the London Symphony Orchestra playing Mahler's Symphony No. 2 in Ely Cathedral. Cooper recreated that scene with the London Symphony in Ely Cathedral.

"The first day, I messed up," Cooper said at a press conference in Los Angeles late Sunday. "I kept getting behind tempo. I was forgetting where the time change happened."

Cooper worried that if he could not portray the magnitude of Bernstein conducting that piece, the movie would fail to show why the composer and conductor was so important. On the second day of filming at Ely, Cooper realized it should be a single take, not all the camera setups he had planned the day before.

"He said, 'You did not conduct us yesterday. We were very good, so we're fine. But today, you just conducted us and you have to use that,'" Cooper said. "He goes, 'You can't use yesterday.'"

Cooper assured the drummer that he was only using the good take. Filming that scene came at the end of a six-year process that began before Cooper's directorial debut, A Star Is Born, came out.

Cooper began researching Bernstein, the composer of songs for On the Town and West Side Story, and original operas like Candide. Cooper zeroed in on the story of his marriage to Felicia Montealegre (Carey Mulligan).

Bernstein was bisexual and continued to date men while they were married.

"That's what seemed very fascinating, this unorthodox, mysterious, also very open, wistful, haunting, funny relationship," Cooper said.

Mulligan, 38, met the surviving members of Bernstein and Montealegre's family. Cooper said their children told him, after meeting Mulligan, that, "She's perfect."

Mulligan also visited Montealeger's extended family in Chile. Mulligan said what stood out, and what the film portrays, was "the sense of fun of their family, that everything was a game and everything was an inside joke."

"It's just a family you wanted to be in," Mulligan said.

While co-writing the script with Josh Singer, Cooper also began practicing conducting. Cooper started working with dialect coach Tim Monich on Bernstein's voice, which Cooper also said "was terrifying."

"There'd be like five steps back," Cooper said. "I was like, 'I'm never going to get the voice to sound like how I want.'"

All the effort Cooper and Mulligan put into preparing to represent Bernstein and Montealegre was anchored by the one element of Bernstein's that still exists, Cooper said. Using Bernstein's music for Maestro's score was what Cooper called "nuclear power."

"If you have nuclear power, you have a shot at making something special," Cooper said. "A Star Is Born, it was Gaga's voice. For this, it was Leonard Bernstein's music."

Maestro premieres Dec. 20 on Netflix.