Little is known of collaborators like Saul, though they left their own records of the atrocities they helped commit.
"When I came across the writings of the Sonderkommando in the Auschwitz scrolls I was transported into the middle of it," Nemes recalls.
"These men were forced to burn their own people and they made writings before dying. They knew they would be liquidated and put those notes into the ground. Those notes were found after the war. They described their daily tasks like taking out personal belongings and sorting them out, as well as how they put together a certain form of resistance."
In the film, Saul seeks a kind of redemption by adopting the corpse of a boy as his own son and attempting to give a decent burial.
"Saul is more involved in making sure his soul survives than his body, and the way you nurture your soul is to give to somebody else," Nemes says.
"So he wants to bury this boy, he wants to do something for someone else. It's a selfless orientation."
It took five years for the Budapest-born Paris rehashed Nemes to bring Son of Saul to the screen. He keeps dialogue to a minimum against a background of eerie industrial camp sounds.
"When the image is not there to give you a lot of information the sounds can say much more. It's a huge machine; it's a like beast. It's moving and never sleeps. It has a dynamic quality to it. It's living, the death is living there."
The film barely strays from Rohrig's Saul. The New York-based Hungarian actor has some sympathy for his character's predicament.
"What choice did they have? Once they figured out what their work would be, some did commit suicide, but others felt strong enough to go on and try to survive. The only way to do it was to desensitise your whole being - to not see what you see and to not hear what you hear and to go through the day minute by minute."
Lowdown
What: Son of Saul, Best Foreign Language Oscar front runner
Where and when: At selected cinemas from Thursday.
