As with Promising Young Woman, Maestro, now streaming on Netflix, would prove to be a transformative experience for her understanding of herself as an actress and what she is capable of. And it is likely to earn her another Oscar nomination and possibly her first win.

Mulligan had become used to only ever getting a few months of prep for most of her roles. Suddenly she had years, which was both wonderful and daunting after Cooper approached her about the role in the summer of 2018.

In addition to learning everything she could about Montealegre and even giving herself over to “dream work” with Cooper, she immediately got to studying her character’s unique dialect. Montealegre was born in Costa Rica, raised in Chile in a multilingual household and educated at British school in Santiago.

Luckily, there were long recorded interviews that Mulligan listened to over and over. She also worked closely with famed dialect coach Tim Monich, who had actually met Montealegre once.

“She was just as you see her in [journalist Edward Murrow’s television show] Person to Person interview. I was a rube-ish 26-year-old and I had never met anyone as easy, relaxed and elegant as she,” Monich wrote in an email. “In a room with her, what a voice. And Carey has that, too … Carey’s voice is one of her glories, and over quite a few roles we have played and altered it for different characters.”

His process, Monich wrote, includes something he calls Language Lab, “in which we listen to the real voices and then imitate them, riff on them and play with them until they are the actors’ ‘real’ voices, too”.

Maestro spans decades and would require Mulligan to play her from age 24, in 1946 when she met Bernstein at a party, to her death in 1978 at age 56. Hair and make-up would thus have to make Mulligan, 38, look both younger and older as well as depicting her evolving, and famous, clothing and hair styles through the ’50s, ’60s and ’70s.

In the early scenes, shot in black and white and on film, she learned that costumes that stood out in that format might not actually look great in real life – but Oscar-winning costume designer Mark Bridges (The Artist) was proficient in both. Her favourite of the many looks, though, was probably the dramatic blue dress she wore for scene featuring the performance of Bernstein’s work Mass.

“It was the best dress for walking away,” Mulligan says. “I was like, this is amazing because she just can’t bear to be there to watch him receive the accolades.”

It was an interesting experience seeing herself age, through make-up and prosthetics, too.

“I absolutely loved being in my late 50s. I don’t know why, I just thought, ‘This is probably my sweet spot.’ Like, I think this is where I feel in my soul,” she says.

“The make-up was so unbelievably real, but it sort of presents this slightly odd mortality crisis that you look at yourself go, well, this is exactly what I will look like if I ever got incredibly sick. It’s a little trippy.”

One time it even confused a doctor who had come to set to prescribe her some antibiotics and did not believe that she was 12-weeks pregnant, asking her “are you sure” and “how do you know?”

“I was like, why is this doctor being so weird? Then I looked in the mirror and realised ‘Oh, it’s because Sian [Grigg] and Duncan [Jarman] made me look 56 and it would have been a miracle baby,’” Mulligan says.

“I went to make-up and said, ‘A doctor has just looked at me and thought I was in my late 50s. So kudos because that’s pretty good make-up.’”

For Mulligan, a review that calls her “lovely” is just about the worst thing that a person can write. She was sad to learn that that is exactly how most critics described Montealegre’s acting.

“She got a lot of ‘she’s lovely’ and this sort of patronising, mediocre, middle-of-the-road reviews,” Mulligan says. “I just thought, oh, gosh, you know, to be married to Leonard Bernstein, who’s like touched by God and then get a review that’s, like, ‘she’s fine’.”

But something else resonated too. On one of the recordings, Mulligan says, “she was talking about the Actors Studio and she was saying she finds it just so embarrassing that these actors are throwing themselves around crying and telling all their secrets and pretending to be animals and how ridiculous.”

She has a theory that like herself, Montealegre was just nervous to fully commit because she did not want to fail. Maestro gave Mulligan the space to finally give herself over to it.

“She just never could quite do it. That really hit me.”