The actress - who recently gave birth to a baby daughter, her second child with Tim - also said Johnny helped her remember her lines because she was pregnant when they shot the film.
She said: "Johnny was very thoughtful because I was pregnant and when you are pregnant, for the first three months it's difficult to concentrate on anything - all your energy goes into the baby.
"Sometimes he was off camera and when I had completely forgotten what Tim had told me, Johnny would just sign language, 'Look over there!' or whatever it was I needed to do, so that was particularly helpful!"