Gene Kelly insulted Debbie Reynolds for not being able to dance. Fred Astaire, who was hanging around the studio, found her crying under a piano and helped her with her dancing.
Only 19 when cast to play the film, Debbie Reynolds lived with her parents and commuted to the set. She had to wake up at 4:00 a.m. and ride three different buses to the studio; sometimes, to avoid the commute, she would just sleep on the set.
For the "Make Em Laugh" number, Gene Kelly asked Donald O'Connor to revive a trick he had done as a young dancer, running up a wall and completing a somersault. The number was so physically taxing that O'Connor, who smoked four packs of cigarettes a day at the time, went to bed (or may have been hospitalized, depending on the source) for a week after its completion, suffering from exhaustion and painful carpet burns. Unfortunately, an accident ruined all of the initial footage, so after a brief rest, O'Connor--ever the professional--agreed to do the difficult number all over again.
http://m.imdb.com/title/tt0045152/trivia
Even knowing this truth about Gene Kelly has not spoiled the clip of him singing "Singing in the Rain" while dancing down the street with the umbrella. It's one of my all time favorite pick-me-up tunes. So happy & well done. 💕