From my favorite movie, Casablanca(1942). Knowing that the actors' and actresses' reactions are genuine because of what they'd actually been through in real life, this scene always gives me chills:
(at around 1h 12 mins) During the scene in which the "La Marseillaise" is sung over the German song "Die Wacht am Rhein" ("The Watch on the Rhine"), many of the extras had real tears in their eyes as a large number were actual refugees from Nazi persecution in Germany and elsewhere in Europe and were overcome by the emotions the scene brought out. The scene was copied from Jean Renoir's The Grand Illusion (1937), in which French soldiers in a German POW camp sing the song as a similar gesture of defiance. In that film the song was led by a prisoner who was in drag for a show the prisoners were putting on. The song was written in 1792 by Claude Joseph Rouget de Lisle in Strasbourg after the declaration of war by France against Austria, and was originally titled "Chant de guerre pour l'Armée du Rhin" ("War Song for the Rhine Army").