The play, titled "The Audience," already won audiences in London before it opened on Sunday at the Gerald Schoenfeld Theatre on Broadway. The play highlights the relationship between Queen Elizabeth II and the 12 prime ministers that have served during the queen's 60 year reign, according to NPR.

A Buckingham Palace officer called "The Equerry," fills the audience in and sets up the play, by saying: "Every Tuesday, at approximately 6:30 p.m., the queen of the United Kingdom has a private audience with her prime minister. It is not an obligation, nor is it written into the constitution. It is a courtesy extended by the prime minister to bring her majesty up to speed."

Morgan researched for the play's material, but since minutes are never taken at the meetings between the queen and the prime ministers, the dialogue is theoretical.

"That covenant of trust and silence between prime minister and sovereign is really, really guarded," Morgan told NPR. "I mean, famously, most of them don't even talk to their wives about it."

"When I was originally studying her," Mirren said, according to NPR, "I stopped looking at any film of her after she became queen. And I only looked at the film of her before she became queen to see who the person was before that mantle fell upon her."

Stephen Daldry, the play's director, has - like Mirren - met the queen several times. "And she's always been the model of decorum and decency - but on reserve," Daldry told NPR. "You know, she is the most invisible visible public woman in the world."

Queen Elizabeth II herself has not seen the play, but many within her inner circle have, according to NPR. "Almost all of the royal household came to see it," Mirren said, according to NPR. "All her ladies-in-waiting came and they all loved it. We got big thumbs up; they all said 'Yes, that's what it's like.'"

"The Audience" will run on Broadway until June. Soon after, Queen Elizabeth II will earn the distinction of being the longest reigning monarch in British history. (Queen Victoria, who served 63 years and 216 days, is the current record holder). If Ed Miliband defeats David Cameron in general elections in May, the play could include a 13th prime minister.