The piano pieces heard throughout the film are the work of Frédéric Chopin, a Polish composer and piano virtuoso, who is regarded as one of the greatest musicians in history.
The hat bought by Benji is a traditional Polish highlander (Gorale) hat. It is worn by men living in the mountains in southern Poland.
Jennifer Grey, who is Jewish-American, was overwhelmed on visiting Majdanek and couldn't stop crying. Jesse Eisenberg initially kept some of her reaction in the film, but ultimately decided it "disrupted" the tone.
The characters in the film visit the German concentration camp Majdanek near Lublin. Between 1941 and 1944, the Germans murdered about 80,000 people there, 59,000 of whom were Jews. The remaining victims were Poles, mainly members of the intelligentsia and the resistance movement. About 6% of the victims were children under the age of 15.
Jesse Eisenberg wrote the role of British tour guide and historian, James, for his friend, actor, writer and director Richard Ayoade. Ayoade thought he would have been a bad fit for the role and his presence would have been too distracting for a serious drama like this so he suggested Eisenbeg offer it to Will Sharpe instead.