The British star plays the troubled sister of a sex addict in the upcoming drama and the role required her to peel off onscreen for the first time.
She was away in Australia shooting scenes for The Great Gatsby when the movie opened in the U.K. so she asked her mum to attend a screening instead - and the actress was left surprised at her response.
Mulligan tells British morning show Daybreak, "I've done lots of costume dramas and (played) comfortable, middle-class teenagers, so it was good to do something different.
"I was in Australia filming The Great Gatsby so I haven't been in London. I sent my mum to watch it. She sort of (responded with) gasps and giggles."