The Roberts are among a host of Hollywood actors starring in the film, including Jennifer Garner, Jessica Alba, Jessica Biel and Bradley Cooper.
But instead of hanging out with the star-studded cast when she wasn't needed for filming, Emma found herself looking after her little cousins - five-year-old twins Hazel and Phinnaeus, and Henry, two.
Emma tells Us Weekly magazine, "I didn't have scenes with her, so I babysat the kids while she was on set. She really is just a mum, at the end of the day. She's totally normal, which I love."