Throughout the movie Esti Kuperman (Rachel McAdams) is shown wearing a wig. This wig is called a sheitel and is worn by some Orthodox Jewish married women in order to conform with the requirement of Jewish law to cover their hair.
Director Sebastián Lelio on how he remembers his first encounters with Rachel Weisz and Rachel McAdams: "The first day with both was a milestone. I was nervous because, deep down, I did not know if there was going to be chemistry between them. I was at the end of a restaurant talking to Rachel McAdams and from afar I see Rachel Weisz walking. She sits down and they start talking. Immediately I realized that there was going to be tremendous electricity between them. The fact that they were so different was going to work perfect for the game of attraction and magnetism that the movie demanded. From my perspective, seeing them both was a sort of epiphany. I saw there was a movie, it was going to be vibrant and urgent. I realized that it was going to be tremendously powerful to watch the acting duel between them."
Director Sebastián Lelio on how different Rachel Weisz and Rachel McAdams are: "The first sensation I had when working with Rachel Weisz was that I was facing a force of nature, someone of volcanic personality. On the other hand, Rachel McAdams is very meticulous. She studies a lot and is something like an expert in disguise, hiding behind the wig and makeup. It seems to me that, in the end, [McAdams] handled all the complexities of her character with an unique elegance. They are very different and fit right into the characters, who are complementary and counterparts at the same time."
The trivia item below may give away important plot points.
In the penultimate sex scene between Ronit and Esti, both women were filmed experiencing orgasm. Director Sebastián Lelio decided to show only Esti's climax, as he felt that it signified a more dynamic shift in character. Rachel Weisz agreed, "Esti's [climax] was more important, and Ronit's robbed her of that."