Speaking to HuffPost UK about getting into character, Emma revealed she did lots of research about Diana, including speaking to people who knew her, which is where the actor discovered a “filthy” tidbit about the late royal.
“I recently met someone who knew her, and who worked with her very closely, and he said that she used to send him the filthiest birthday cards,” Emma said.
“He used to say ‘darling where have you found these?’, and she used say, ‘I know people’, which I think is brilliant.”
She continued: “He would say, ‘please tell me you’re not going into shops and buying these’, but she was like, ‘oh yeah, there’s a little shop down the road from me, so I go in’. And I love that about her, I love her sense of fun.
“I think that’s something I’d always known was there, because I think that’s why so many people are so drawn to her, and why they felt like she was such an un-royal person, I suppose.”
Gillian Anderson is joining as prime minister Margaret Thatcher, while British comedy actor Rebecca Humphries will appear as her daughter (and future I’m A Celebrity champion), Carol Thatcher.
The Crown will return for its fourth series on Sunday 15 November. Read our full interview with Emma Corrin and Josh O’Connor.
This article originally appeared on HuffPost UK and has been updated.