With a budget over $10 million, this is one of the most expensive movies made in Denmark.
The movie takes its title from the nicknames of the two main characters. Flame refers to the color of Bent Faurschou-Hviid's red hair, after an attempt to dye it blond. Citron (Danish for "lemon") got the name because, while working at the Citroën factories in Copenhagen, he sabotaged German cars and trucks.
Almost every time we see a cigarette being lit on-screen, it is lit by The Flame.
Based on actual events that occurred in Denmark during World War II.
The two lead actors, Mads Mikkelsen and Thure Lindhardt, had previously acted together in "Rejseholdet" Assistancemelding A-7/01, del 2 (2002).