Casablanca has such a strong storyline. But what really makes it a classic are the characters. The characters are so solid and each role was cast brilliantly in my opinion.
Rick is the main character and his sarcastic and dry tone was fun even while his serious and sad face stayed straight. He rarely smiled in the film, and even came across a bit harsh at times but I found myself immediately rooting for him. Rick is very bitter- but strong. Ilsa was sad but strong and proud. The love triangle must have been a reason for the films instant acclaim.
The humor that the town Captain (Renault) brought was throughout the film. I really enjoyed this character. He was a true neutral character until the end where he let Rick off the hook.
Sam was another great supporting character that had great expressions and was genuine and lovable.
The characters were the driving force that made the already good plot into a classic.