Peter Morgan has told the Queen’s story in other mediums—the 2006 film The Queen and the 2013 play The Audience, both of which starred Helen Mirren as the monarch. Did Peter put you in touch with Helen before filming to get her feedback on playing the character?

I would have loved that! I would have loved it if he’d said, ‘Would you like to come meet Helen Mirren?’ I would have been like, ‘Yes.’ Not even to talk about the Queen necessarily . . . but to talk about life. I think we should all have a private audience with Helen Mirren.

No, but The Audience was still [in production] with Kristin Scott Thomas on the West End [when I was cast]. [Peter and Stephen] were like, ‘Let’s go see it together.’ And I said, ‘I really want to see the show but I don’t think it would be a good idea now to see Kristin Scott Thomas play the Queen, because I might feel slightly intimidated.’ I steered clear of that. But I did watch The Queen again when I first found out I got the job. I pretended I was doing research.

Did you speak to anyone who had worked with the Queen in preparation?

We talked to people who’d worked for the palace, but it’s a funny thing: people are very loyal and [what happens inside the palace] is very shrouded in secrecy. They are very professional, and it was difficult to get behind the veneer.

Was there anything they told you that was surprising?

Unfortunately not. But I was surprised with how strict they still are on protocol and on people observing the etiquette. [The palace is] a pretty modern household now, and I thought they would be, ‘Oh no, you don’t have to call me that,’ or, ‘Don’t curtsy. How embarrassing.’ But they’re very, very strict on it still. I think they see protocol and etiquette as not something to be revered or anything like that, but it’s very important to the continuity of the monarchy.

I think there is very much a sense of once you served, you always serve, in a way.

Has your posture improved since playing the Queen?

I think it has. I’d just had a baby when I started filming, so I had to wear a proper corset because I was about five dress sizes bigger than I normally am. The corset helps you not slouch. Now we’re doing the second series. I’m not wearing it anymore, but it stays with you, that posture, and being a lady.

The scenes between Queen Elizabeth and Princess Margaret (played by Vanessa Kirby) are especially hard and heart-wrenching to watch—the Queen having to hurt her sister by not letting her marry the love of her life. What was it like filming those?

It’s interesting, because Vanessa and I had never really spoken about it until a few days ago. I knew very much what Margaret felt about the Queen and how much she felt she’d been let down . . . I do feel bad [for Elizabeth] because I think nothing hurts more than hurting your own sister, and that is true for everybody, especially if you’re an elder sibling. The idea of your younger sibling being in pain and realizing you are the cause of that pain is unbearable. But in order to make her happy, Elizabeth would have had to go against everything that she held as a core belief—she would have also had to betray her father, betray God. She was no longer in a position where she was able to do that.