Symbolism is the language of all the arts. A symbol can be an image that points to something other than itself, such as a bald eagle being a symbol for the USA. Often, the best way to describe something is indirectly, through symbolic references which might be more familiar to the reader or viewer. Symbols can also help underscore the story’s themes or mirror the character’s inner life.
In film, symbolism also includes metaphor, an expression or figure of speech used to describe something else. For example, one might say “I just got a new set of wheels” to describe purchasing an entire car.
Another symbolic storytelling device is allegory, which can be a smaller story within the larger one that encapsulates or sheds light on the main theme. In Casablanca, the young Bulgarian newlyweds’ plight – and their desire to go to America – serves as an allegory for the whole movie, particularly the love story between Rick and Ilsa.
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Also, an entire film can be an allegory to highlight something that is going on in the ‘real’ world. Casablanca itself is an allegory for what was happening early on in World War II, including America’s reluctance to enter the war, which was represented by Rick Blaine’s avowed neutrality.