The Queen depicted the monarch as she navigated through the grief and series of events which followed as a result of Princess Diana’s death in 1997.
It explored the Royal Family’s response, as well as the outcry from the general public, with Mirren playing the titular role of Queen Elizabeth.
The 76-year-old added: “How do you write to your Queen? Was it Madam, or Your Highness, or Your Majesty?
“I said, ‘We are doing this film. We are investigating a very difficult time in your life. I hope it’s not too awful for you.’”