Gene Kelly
Gene Kelly's "Alter-Ego" scene from Cover Girl (1944) is a mind-bending practical effect in which Kelly dances with his own ghost. The camera operator had to pan and dolly down the street at exactly the same speed for two separate takes — if the camera lagged by even a fraction of a second, the two Kellys would drift apart and the illusion would fail. The "leapfrog" moment required Kelly to jump over a darkened ladder, calculating height and timing so he would clear his ghost's head in the final composite. The effect was so difficult the film's director walked out; choreographer Stanley Donen then spent over a year in post-production optically combining the two moving shots into one seamless piece of movie magic. In the climax, Kelly throws a trash can through a real window that was not safety glass — he performed the stunt without injury. It was Kelly and Donen's first major collaboration; Donen had been a Broadway chorus dancer discovered by Kelly, who brought him to Hollywood as his assistant.