Doing It for the Art: Gene Kelly was sick and feverish the day they shot the iconic title sequence, but he powered through and sang and danced at full capacity.
Executive Meddling: The studio felt there had to be a musical number in which the titular song was sung. Hence, the Singin' in the Rain sequence with Gene Kelly literally singing in the rain.
Scully Box: Gene Kelly choreographed his dance scenes with Cyd Charisse to hide that she was taller than he was. To keep the height difference from being obvious, Kelly arranged the routine so that they were never both standing upright when they were next to each other, always bending toward (or away from) one another instead.
Star-Making Role: For Debbie Reynolds, who was 19 when this movie was made. Invoked by the studio, who felt she was ready to be pushed to the next level of stardom.
As Singin' lore reveals, it was a bumpy ride for Debbie Reynolds. Although Reynolds and Gene Kelly's characters (eventually) became sweethearts on screen, in reality the two actors did not get along and frequently bumped heads (not in the least because she had not been his first choice, as she had to learn not just to do the dances for the film but to dance, period, in just three months). In fact, Kelly, in his role as director, once mocked Reynolds' dancing to the point where, after shooting finished, Fred Astaire (who was visiting the set that day) found her huddled under a piano sobbing. He helped her get her dancing closer to Kelly's draconian standards. Kelly also made Reynolds tap so much for the number "Good Morning" without any break that her feet began to bleed and she needed to be carried to her dressing room. In the end, Kelly hated how Reynolds' tapping came through he dubbed over it. Bear in mind that Reynolds was entirely new to tapping until Astaire came along and helped her.
She wasn't the only one to have problems. Kelly spent three days filming the title number while running a high fever, and Donald O'Connor was so exhausted after one of his big numbers that he needed four days of bed rest afterwards.
An early draft of the script featured the title song as a number for all three of the leads, not just Don.
An un-filmed alternate ending reveals that Don and Kathy eventually married, and continued acting in movies together. It also reveals that Lina married Cosmo, and landed movie roles that wouldn't require her to deliver a lot of spoken dialogue (such as an unintelligible Jungle Princess).
Originally, Debbie Reynolds was going to play Gene Kelly's partner in the "Broadway Melody" sequence, but her dancing wasn't up to the task. Leslie Caron, who had danced with Kelly in An American in Paris, was the second choice, but she was unavailable.
In the first draft, Zelda Zanders was to sing "I Got A Feeling You're Fooling", but after script revisions the song was used in the montage before the number, "Beautiful Girl", along with "The Wedding Of The Painted Doll" and "Should I".
Originally, Kathy was to sing "You Are My Lucky Star" to a billboard of Don Lockwood after he sang to her in the studio, by way of dramatizing that she was the president of the Don Lockwood Fan Club. It was shot, but didn't end up in the finished film. However, it's available on the film's original soundtrack and DVD, as shown in this clip.
Very early on in the pre-production stage, Judy Garland, June Allyson and Ann Miller were considered for Kathy Selden, but they were all considered "too old".