Alain Delon, like many French, had admired Kelly el ametralladora (1958), one of the first Bronson's starring vehicles, and he had the producer approach him when Bronson was in Europe making Villa Rides (1968).
For the French soundtrack, Charles Bronson's voice was purposely dubbed with a very strong American accent by director John Berry.
In the doctor's office there is a calendar that says Dec. 20th, Mercredi, so assuming it's accurate this was filmed in 1967.
Charles Bronson's agent Paul Kohner later recalled: "Silberman pitched Bronson on the fact that in the American film industry all the money, all the publicity, goes to the pretty boy hero types. In Europe, he told him, the public is attracted by character, not face. Bronson had always resisted doing European films before ... This time he was only half convinced by Silberman's arguments, but I made the deal for him to do Adieu l'ami." Bronson was signed in December 1967.