Jean Gabin
Jean Gabin stars as Lieutenant Maréchal, a French soldier and man of a lower class. Before starring in The Grand Illusion, Gabin starred in a number of films—some of which are considered among the best films to have ever been produced. Gabin was considered the biggest office draw in France, and Renoir was only able to secure financing for his film after he secured the services of Gabin as his lead actor. It was Gabin, in fact, that Renoir had in mind when writing the script for the film.
Director Jean Renoir was very happy working with Gabin. He called the actor a "true film actor" and said that he had "never met such a cinematic power; he's a cinematic force, it's fantastic, it's incredible. It must come from his great honesty. He's certainly the most honest man I've ever met in my life."
Marcel Dalio
Marcel Dalio stars as Lieutenant Rosenthal, a French soldier from a well-to-do Jewish family. Before his starring turn in The Grand Illusion, Dalio starred in a few minor French films. The Grand Illusion is the film that Dalio is arguably best known for and was the film that inspired and long and successful career for Dalio.
As a Jewish man, Dalio had a lot to say about Rosenthal, who is a Jewish man. And Renoir was eager to hear the actor's suggestions. He worked collaboratively with each of the actors in the film; Renoir's collaboration inspired Dalio to work with the director again in his follow-up to The Grand Illusion, called Rules of the Game.
Pierre Fresnay
French actor Pierre Fresnay stars as Captain de Boëldieu, a French flying ace, soldier, and member of French nobility. Although Fresnay was best known as a stage actor, he starred in a number of films as well, making the form familiar to him. The Grand Illusion was no doubt the most famous film he starred in.
Dita Parlo
German actor Dita Parlo stars as Elsa, the woman with whom Maréchal falls in love at the end of the film. Though Parlo's role in The Grand Illusion is relatively small, her presence is felt throughout the film. Before the release of The Grand Illusion, Parlo starred in a dozen or so films. Among those, she was best known for her role in Street of Shadows. However, The Grand Illusion quickly became the role for she was best known.
Erich von Stroheim
Austrian-American actor Erich von Stroheim stars as Captain von Rauffenstein, a German soldier, flying ace, and member of German nobility. Despite being cast as the film's German villain, von Stroheim did not know how to speak German prior to (or mostly during) production. Throughout the production of the film, von Stroheim worked hard to learn his lines and the German language in between takes. This led to clashes between von Stroheim and director Jean Renoir, but von Stroheim ultimately turned in an award-worthy performance (and von Stroheim and Renoir became friends after).
von Stroheim started his career as a director and made some of the most well-respected films of all time. However, people found it hard to work with him and he became an actor who worked primarily in French cinema. Though von Stroheim went on to star in a number of very impressive films, he is best known for his work in The Grand Illusion.