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, ended up in a hospital bed for a week after its completion. He suffered from exhaustion and painful carpet burns. Unfortunately, an accident ruined all of the initial footage, so after a brief rest O'Connor agreed to do the difficult number all over again.
Gene Kelly was a taskmaster with Debbie Reynolds, who had never danced to this degree before rehearsals started. Fred Astaire, who was in an adjacent dance studio, found her crying under a piano and reassured her that all of her hard work was worth the effort.
A microphone was hidden in Debbie Reynolds' blouse so her lines could be heard more clearly. During one of the dance numbers, her heartbeat can be heard, mirroring what happens to Lina Lamont in the movie itself.
Donald O'Connor recalled, "I was smoking four packs of cigarettes a day then, and getting up those walls was murder. They had to bank one wall so I could make it up and then through another wall. We filmed that whole sequence in one day. We did it on a concrete floor. My body just had to absorb this tremendous shock. Things were building to such a crescendo that I thought I'd have to commit suicide for the ending. I came back on the set three days later. All the grips applauded. Gene Kelly applauded, told me what a great number it was. Then Gene said, "Do you think you could do that number again?" I said, "Sure, any time". He said, "Well, we're going to have to do it again tomorrow". No one had checked the aperture of the camera and they fogged out all the film. So the next day I did it again! By the end my feet and ankles were a mass of bruises."
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.