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).
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.
Warner Bros. edited the movie's trailer to remove Esther's line, "It must be hard to love an adopted child as much as your own," after receiving numerous complaints from adoptive parents and foster care organizations. However, the line remains in the film.
The movie was originally written to take place during the fall, and some exterior shots were filmed to establish this. Shortly before principal photography was set to begin in December 2007, the Toronto locations were hit with near-record snowfall, forcing the change of setting to winter. One of the scenes written out as a result was a Halloween carnival at Daniel and Esther's school.
When Kate is writing and playing music on her piano, she is actually composing a suite for Max and Jessica. This scene was originally longer, and Vera Farmiga, herself a skilled pianist, was reportedly furious at director Jaume Collet-Serra for editing it down. She hoped the full scene would be included on the DVD (it was not).
The orphanage Esther is adopted from is "Saint Mariana's Home for Girls." In the Catholic faith, Saint Mariana of Quito is the patron saint for those that have been rejected by religious orders and those who have lost both parents.
According to the original script, it was strongly implied that Esther enjoyed harming and torturing animals. A minor subplot with Esther killing and dismembering the Coleman family's pet hamster was written, but never filmed.
The language spoken by the Saarne Institute receptionist is Estonian. She says, "Saarne Instituute, kas ma saan teid aidata?" (Saarne Institute, can I help you?) "Kuidas palun?" (I'm sorry?) "Ma ei saa aru. Oota üks silmapilk." (I don't understand. Wait a moment.)
The trivia items below may give away important plot points.
Earlier drafts of the script included more information about Esther's past and explain why she attempts to seduce her adoptive fathers. She was molested by her father for years, starting when she was an infant, and this sexualized her at a very young age and destroyed any future chance of her having her own children. Her father later took another lover, telling Esther that, because of her condition, she could never be a real woman. She murdered them both and was ultimately sent to Saarne, the mental institution. After escaping from Saarne, she worked as a prostitute in Estonia for years, mostly catering to wealthy pedophiles. When she was arrested for this, she kept up the pretense of being a child to stay out of jail and was sent to an orphanage. Esther sees herself as trapped inside the body of a child, and it disgusts her. She wants to "grow up" and be a wife, a mother, and a lover (what her father considered a "real woman"), and tries to find "love" where she thought she had had it as a child, with her new father.
In earlier drafts of the script, Kate and John go to the orphanage not to see girls for the first time, but to bring presents to Yolanda, a seven-year-old Puerto Rican girl whom they plan on taking home the next day. There, they meet Esther (this scene plays out almost exactly as it does in the finished movie). Although she impresses them, especially John, she finds out that they have already planned to adopt Yolanda. The next morning, Sister Judith finds Yolanda hanging in a closet; it is assumed that she died in a kids' game gone wrong, but implied that Esther killed her. John later suggests to Kate that they adopt Esther instead.
An alternate ending for this movie appears on the DVD version. Esther is not killed by Kate at the pond but is instead up in her room putting on make-up, once again assuming the role of Esther as she greets the police.
In earlier drafts of the script, Esther does not stab John to death. Instead, as he discovers the black light paintings in her room, she jumps out from beneath the pile of stuffed animals on her bed, stabs him in the eye with a pair of scissors, pushes him down the stairs (breaking his leg), and strangles him with a jump rope.
In earlier drafts of the script, Esther pins the murder of Sister Abigail on a homeless man in the park by hiding a grocery bag containing the bloody hammer and personal items from Abigail's car among the man's possessions. This was filmed, and part of it is featured as a deleted scene on the DVD.
The climax of the movie has an uncanny similarity to the climax of The Ring Two (2005). In that film, Naomi Watts' character gets out of the well and suddenly, Samara, the antagonist, calls her "Mommy," to which Watts replies to her, saying," I am not your f***ing mommy." She then blocks the well with a boulder. In this movie, Vera Farmiga's character gets out of the water and Esther says, "Please don't kill me, Mommy," and Farmiga's character replies, "I'm not your f***ing mommy!" She then kicks Esther to fall back into the ice cold pit.
The movie closely resembles the real-life "Kurim Case," where a woman, Barbora Skrovlá, was born with a strange type of hormonal disease that made her look like a child. Skrovlá was in the Mauerová family with the name "Anna," and she committed several acts of cruelty. The resonance of the case was Klara Mauerová, the mother of two children, who committed acts of extreme abuse, according to Klara, under "Anna's ideas" and for protecting "her."