Although uncredited, Gene Kelly had two incredibly talented choreography assistants for Singin’ In The Rain. These ladies were none other than Carol Haney (The Pajama Game (1957)) and Gwen Verdon (Broadway star of “Can-Can”, “New Girl In Town”, “Damn Yankees”, “Redhead”, “Sweet Charity” and “Chicago”).
In fact, Kelly’s taps during the “Singin’ In The Rain” number were post-dubbed by Verdon and Haney. The ladies had to stand ankle-deep in a drum full of water to match the soggy on-screen action. Gene Kelly had also recommended Carol Haney for the role of Kathy Selden (but Debbie Reynolds got the part!)
Studio technicians had to cover two outdoor city blocks on the backlot with tarp to make them dark for a night scene, and then equipped them with overhead sprays for Gene Kelly while he was ‘dancing in the rain’. Their efforts are all the more remarkable since there was a severe water shortage in Culver City the day the sequence was shot. –IMDB