Director Jacques Becker used mainly non-actors for purposes of authenticity. In fact, one of his choices was actually involved in the 1947 escape.
The scene where three different characters take turns breaking through the concrete floor of their cell is filmed in a single, nearly four minute long, shot.
La Sante prison was replicated right down to the smallest details, thanks to the help of the three actual members of the escape whom Jacques Becker hired to serve as production consultants.
The soundtrack to this film is entirely Diegetic (the source of every sound is visible, or occurs on screen)