The fort that is posing for Fort Vaux during the attack on it in the film is actually Fort Tavannes. It is not far from Fort Vaux, and has been attacked during the real battle in 1916.
According to the director, Léon Poirier, the entire cast and crew of the film were made up of Great War veterans. The only exceptions being female actresses, as Suzanne Bianchetti and Jeanne Marie-Laurent, and older actors such as André Nox and Maurice Schutz. Antonin Artaud also wasn't a veteran.
The scene in which the Intellectual (Antonin Artaud) dies, is actually filmed in a real shell hole. Remains of a human arm were found with a ID tag on which the filmmakers could just make out: "Fulle... 1916" (Class of 1916).
The film is filmed on location in Verdun and surroundings.
For the Bois des Caures scene many actual veterans are used. Among them are Lieutenant Robin and captain Vantroys, who responded Léon Poiriers call to participate in the film. They are playing themselves in the film.