Seeing a group of German soldiers in Rick's bar singing the song Die Wacht am Rhein, Victor Laszlo gets the band to play La Marseillaise (with Rick's nodded assent). Laszlo and all the other customers sing their hearts out, completely drowning out the Germans.note Watch the man at the back of the German group as they try to continue their song. At one point, he slips and sings "L'étendard sanglant est levé". The final touch is when Yvonne (who had previously been flirting with a German soldier) starts weeping as she sings, and shouts "Vive la France! Vive la démocratie!" after the song is over.
And remember, this movie was produced and premiered during World War II. Those expatriate actors aren't remembering those times, they were living those times.
Moreover, there was a genuine concern at times that the US could either lose the war or withdraw from it, and deport these actors back to Nazi-occupied lands. There was a serious possibility they could be executed for their participation in the movie.
Laszlo gets a lot of awesome. His character has already escaped from a concentration camp and then evaded the Germans most of the way across Europe with them hot on his tail the whole time. Even other characters in-universe are impressed. But his best awesome line is when he's pointing out to Major Strasser that killing him won't stop the resistance:
Laszlo: For every one of us you kill, ten more will rise, and hundreds after them. Even Nazis can't kill that fast.
Notably, this was before most of the world knew of the Holocaust. Some of the cast likely knew, however, including Conrad Veidt, though such comments were usually dismissed as propaganda. It's entirely possible this line was written knowing exactly how nasty the Nazis really were, to show how heroic those who opposed them successfully were, too.
Captain Renault's riposte to Major Strasser: "We mustn't underestimate American blundering. I was with them when they blundered into Berlin in 1918."
Captain Renault has just seen Rick kill Major Strasser. The cops show up and look to him to tell them what's happened. "Major Strasser has been shot," he says. A pause, while Rick looks tensely at him. "Round up the usual suspects."
In one of the most famous I Just Want My Beloved to Be Happy speeches ever as Rick tells Ilsa that they can't be together because the world is more important than they are. His CMoA as he gives up the most important thing in his life for others.
Rick: I'm no good at being noble, but it doesn't take much to see that the problems of three little people don't amount to a hill of beans in this crazy world. Someday you'll understand that. Here's looking at you, kid.
The whole movie is one for Humphrey Bogart, who, given only his second chance to play a heroic protagonist (third if you count High Sierra), and first to play a character who wasn't a total bastard, a chance he only got because other actors turned it down... he turns in arguably the most iconic performance in film history. This pretty much flipped his type casting, and he didn't play another villain until near the end of his career.
Paul Henreid, too, given less screentime and far fewer awesome and immortal lines than Bogie, somehow manages to avoid becoming the Romantic False Lead... which is exactly what happened to every "Victor" in every Casablanca homage/retread/imitation since.