The actual plans of the Auschwitz-Birkenau death camp and its crematoria were used to build the sets as historically accurate as possible. A 90% scale reproduction of Auschwitz-Birkenau's crematoria II and III (in the film counted as I and II) was build near the village of Giten, 45 minutes outside Sofia, Bulgaria. The result represents a fairly accurate depiction of a small part of the Auschwitz-Birkenau death camp.
The director of photography, Russell Lee Fine, shot most of 'The Grey Zone (2001) with a hand-held camera. ''A film like Schindler's List (1993) does this beautiful photography. (...) We're not going to beat that quality. So we've given it an intentionally rough, hand-held look; made the images less romantic and less heroic. We want it to feel like you're there.'' [NYTimes 2001]
This was the first feature film ever about the 'Sonderkommando' at Auschwitz-Birkenau. In 2015 the acclaimed Hungarian-language feature film Saul fia (2015) became the second one. While both films deal with the same historical events including the revolt on October 7th,1944, they are told from different perspectives, in different styles and highlight different themes. The Hungarian film also characterizes the 'Sonderkommando' less ambiguous. Both films are based in part on doctor Miklos Nyiszli's "Auschwitz: A Doctor's Eyewitness Account", but "Son of Saul" (2015) used additional historical accounts, especially "The Scrolls of Auschwitz" [aka "Voices from beneath the Ashes"] (edited by Ber Mark), a collection of secretly written and hidden testimonies by members of the 'Sonderkommando' themselves.
The film's title is inspired by the second chapter of the essay collection "The Drowned and the Saved" by Holocaust survivor Primo Levi, first published in 1986. The second chapter is called "The Gray" (spelling correct) and deals in an analytical way with the subject of the 'Sonderkommandos'. Since Levi was never part of a 'Sonderkommando' in Auschwitz and never met one himself, his knowledge was mostly based on the critical description of Miklos Nyiszli in "Auschwitz: A Doctor's Eyewitness Account". That book is the main historical source of The Grey Zone (2001), too.
This is probably the first feature film to depict a gas chamber in the process of killing people from the inside. It's only shown for a few seconds as part of a montage. The reasons why this is usually avoided in films are of ethical nature, but it's also difficult to show it in a realistic way, because the gassing in the gas chambers was done in total darkness without any light.