The actress had kept the baby news a closely guarded secret on set, but felt she needed to tell the Welsh star she was with child before he started getting physical with her during a scene they shot in December, reported Contactmusic.
"I was one of the first people to find out. There's a little bit of a kerfuffle (fight) and just before, Emily leans into me and whispers, 'I'm four months pregnant!'
Blunt became a mother for the second time in June when she gave birth to daughter Violet.