Acording to an interview to Melchor Miralles, he said that the real "Lobo" visited the set one day, but nobody knew who really was.
Eduardo Noriega had to work hard for several months before the shooting started in order to speak with the strong Basque Country accent required for his character.
Acording to an interview to Mélanie Doutey, she was help by a dialect coach during a month before the shooting, because she could not speak Spanish at the time.