The scene was apparently filmed in one take too! Joaquin Phoenix working on the fly definitely produced some incredible moments and is no doubt the exact reason he was cast in the role. Todd Phillips previously revealed that Arthur Fleck also improvised his dance in the bathroom after killing the men on the subway. It was originally him talking to himself in the mirror, but the actor asked if he could try something else, resulting in the dance. In Sher’s interview with Slash Film, he explained how this was a turning point for the production. In his words:
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