I cry at this scene, every time. I start to cry thinking about it.

From the wiki, which contextualizes what I was told by my parents (who were young adults in London during the war)

Much of the emotional impact of the film has been attributed to the large proportion of European exiles and refugees who were extras or played minor roles (in addition to leading actors Paul Henreid, Conrad Veidt and Peter Lorre): such as Louis V. Arco, Trude Berliner, Ilka Grünig, Lotte Palfi, Richard Ryen, Ludwig Stössel, Hans Twardowski, and Wolfgang Zilzer. A witness to the filming of the "duel of the anthems" sequence said he saw many of the actors crying and "realized that they were all real refugees".[25] Harmetz argues that they "brought to a dozen small roles in Casablanca an understanding and a desperation that could never have come from Central Casting".[26] They were frequently cast as Nazis in war films, even though many were Jewish.