The Brit, who plays eccentric Queen Anne in the royal comedy, admits she struggled to keep a straight face as the Oscar winner pretended to pleasure her.
"We really struggled with not giggling because there's also a camera crew (there)," she tells the BBC, revealing that kissing co-star Rachel Weisz was a lot of fun.
Olivia explains, "I tend to avoid sex scenes as I get embarrassed. We had two weeks of rehearsals. We couldn't be embarrassed in front of each other as we'd rolled on the floor with each other, eaten together, shared everything, and become genuinely good friends."
"So when we had to do this, it was like, 'Come on'. Snogging (kissing) is fine, snogging Rachel Weisz is like you've won the lottery!".