According to Renoir's memoirs, von Stroheim, despite being born in Vienna, Austria (then the Austro-Hungarian Empire) did not speak much German, and struggled with learning the language along with his lines in between filming scenes.
The exteriors of "Wintersborn" were filmed at the Haut Koenigsbourg Castle in Alsace. Other exteriors were filmed at the artillery barracks at Colmar (built by Wilhelm II) and at Neuf-Brisach on the Upper Rhine.
An early script version had Rosenthal and Maréchal agreeing to meet in a restaurant at the end of the war. In the final scene, everyone there would be celebrating the armistice, but instead of these men, there would be two empty chairs at a table.
Renoir was a French aviator during World War I. Gabin wears Renoir's uniform in the film.
In Grand Illusion, director Jean Renoir uses the First World War (1914–1918) as a lens through which to examine Europe as it faces the rising spectre of fascism (especially in Nazi Germany) and the impending approach of the Second World War (1939–1945). Renoir's critique of contemporary politics and ideology celebrates the universal humanity that transcends national and racial boundaries and radical nationalism, suggesting that mankind's common experiences should prevail above political division, and its extension: war.
On the message of the film, Renoir himself said, in a film trailer dating from the re-release of the film in 1958: "Grand Illusion is a story about human relationships. I am confident that such a question is so important today that if we don’t solve it, we will just have to say ‘goodbye’ to our beautiful world."
Source : Wikipedia