Casablanca is, in my opinion, the greatest movie ever made. Most Anglophone film buffs put it in the top three along with The Godfather and Citizen Kane. There are many elements to why it is a great film including Michael Curtiz's direction, the script with it's snappy, witty dialog, and of course the stellar performances by Humphrey Bogart and Ingrid Bergman. And I'm pretty much a sucker for anything that has Claude Rains in it.
But what really made the film for me were the performances by the supporting actors like Madeleine LeBeau. In Casablanca she plays Yvonne, Rick's embittered ex-girlfriend who takes up with a German officer. It's a small role but one that has great emotional punch because during the famous Marseillaise scene she cries out "Vive le France! Vive la democracie!"
She provides that extra oomph that makes an already emotional scene even better. And it is more believable coming from her than possibly some other actress because LeBeau was a wartime refugee.
Casablanca is a story of men and women trying to cope during the horrors of World War 2. Many of the supporting actors in it like LeBeau and her then husband Marcel Dalio, who played Emil the croupier, had fled Nazi occupied Europe for the safety of the United States. It is the verisimilitude of these and other genuine refugees in the small parts of this film that provided it with that little extra something that more than made up for the occasional plot hole.