The 25-year-old actress said their new movie 'Magic in the Moonlight' will show another side of her co-star that no one has seen before, reported Contactmusic.
"I think he tricked us all in believing that he's this charming, lovely fellow. But he's really - you'll see - I think he's coming out of his shell. He's a nightmare," she said.
Firth, who is defined by his role as Mr Darcy in 'Pride and Prejudice', will star alongside Stone as Stanley in the romantic comedy set in 1920s France.
'Magic in the Moonlight' will hit cinemas on July 25.