The Harry Potter and Downton Abbey star, 66, takes over the role of the Queen from Olivia Colman and Claire Foy in season five of the Netflix royal drama, due to begin streaming on 9 November.
When she returned home from filming on set on 8 September to hear the news that the Queen had died, Staunton said she was “inconsolable” and that she had not expected to feel so devastated by her death.
Staunton told Radio Times: “I was very glad I wasn’t due to be working the next day. They couldn’t have filmed on this face.
“The outpouring of grief took me by surprise and might not have been so acute for me had I not been playing her.”
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Although Staunton has won a Bafta and Laurence Olivier Awards throughout her career, she said that the prospect of taking on the role for The Crown had been particularly daunting.
She said: “I knew it was a very heavy baton that was being passed over, so I had to try and match what had gone before.
“I didn’t want to let the whole thing down. I had to get over the shock of doing it and the fear.”
Other cast joining for season five include Jonathan Pryce as the Queen's husband Prince Philip, Lesley Manville as Princess Margaret, Dominic West as Prince Charles, Elizabeth Debicki as Princess Diana, Olivia Williams as Camilla Parker Bowles, and Jonny Lee Miller as John Major.
Pryce, 75, said: “I already knew it was a really good series, as I’d watched it from the beginning – though I started off as a reluctant viewer.
“I’m not a monarchist, and I thought I would have very little interest in the royal family but my wife Kate and I decided to watch it. We got to the end of episode one, and we turned to each other and said, ‘Shall we watch another one?’. We just got hooked."
The Crown season five begins streaming on Netflix from 9 November.