Director Jacques Becker died two weeks after completing the film.
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Director Jacques Becker used mainly non-actors for purposes of authenticity. In fact, one of his choices was actually involved in the 1947 escape.
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Jean-Pierre Melville regarded this as one of the greatest French films ever made.
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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.
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The film was completed according to the director's wishes after his death, but producer Serge Silberman had it reduced from the initial 140-minute version by about 24 minutes to enhance its commercial possibilities. The missing footage remains lost.
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The soundtrack to this film is entirely Diegetic (the source of every sound is visible, or occurs on screen)
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Shot over a period of 10 weeks.
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Catherine Spaak got her big break when her uncredited brief appearance as Nicole in this film caught the eye of a Paris TV reporter. Sophia Loren happened to see the reporter's interview with Catherine on TV. She immediately thought she looked perfect for the lead role in "I dolci inganni" (Sweet Deceptions, 1960) which was being cast by her producer-husband Carlo Ponti.