Based on a true story.
Jean Baptiste Buisson, Jean Louis Trintignant's character's brother in true life - Emile Buisson - did not agree at all that his brother's life was filmed. But Alain Delon met him and Buisson finally accepted against a good check that Delon gave him.
Jean Baptiste Buisson - Emile Buisson's brother - whose nickname in the underworld was Le Nusse, really met André Pousse - who played his character in the film - at Pousse's restaurant Le Napoléon Chaix in Paris. Buisson was in company of Auguste Le Breton, one of his long time friend.
Three of Jacques Deray's friends advised him to take André Pousse to play Jean Louis Trintignant's character's brother. Strangely, Deray decided precisely not to do it, and when he told this to André Pousse, the latest was offended and turned back. But his good friend Alain Delon interfered between him and Deray and Pousse was eventually hired for the role.