Actress Aryana Engineer, who plays Max, their completely-deaf biological daughter in this film, is mostly deaf. She has Cochlear Implants which enhance her abilities to hear and speak.
Isabelle Fuhrman auditioned for the part of Esther while wearing an old-fashioned dress and ribbons around her wrists and neck.
In the screenplay, Esther was described as having fair skin, delicate features, and platinum blonde hair. Although Isabelle Fuhrman (Esther) did not fit this description, the filmmakers were so impressed with her auditions that they cast her anyway.
Director Jaume Collet-Serra allowed Isabelle Fuhrman to do the swearing scene (in which Esther informs Kate that she is aware adults have sex) in one or two takes so she would not have to swear repeatedly.
The subtle uneasiness of the film's poster is due to the image of Esther's face. It is perfectly symmetrical. Half of her face has been mirrored to form a whole face (notice the identical twists of hair on each side).