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.
Citronen (Mads Mikkelsen) and Frode "Ravnen" Jacobsen (Lars Mikkelsen) are real-life brothers and have along side Flammen (Thure Lindhardt) been in two episodes of the danish TV show Rejseholdet (The Unit). Though Mads Mikkelsen was part of the main cast, the two others, were only guest stars. Like this movie, that show was mostly based on real-life events in Denmark.