Donald O'Connor filmed the 'Make 'em Laugh' number in one day, over several takes. By the end of that one sequence, he was so bruised and battered from the repeated unpadded pratfalls, he could barely move. He was unable to work for several days after.
The script for this movie was written after the songs were.
By the time the dance number "Good Mornin'" was finished, Debbie Reynolds had danced for so long and so hard all day long that her heartbeat dropped and she was confined to bedrest.
"Singin' in the Rain" wasn't in the original screenplay. When it was added, it was intended to be performed by Gene Kelly, Donald O'Connor and Debbie Reynolds. This song eventually became "Good Morning", and "Singin' in the Rain" became Kelly's iconic solo.
It has been known that Debbie Reynolds was frequently picked on by Gene Kelly for not being to dance so well. One day while Reynolds was crying on the set, Fred Astaire helped her with the dancing.