Of all the famous dance partners choreographer-director Gene Kelly worked with, Jerry, the superimposed cartoon mouse from 1945's Anchors Aweigh, was his favorite. "He showed up on time and worked his little tail off," Kelly once said.

Never mind that other partners, like 19-year-old Debbie Reynolds, nearly worked their feet off. By now, the grueling footwork that went into Singin' in the Rain's "Good Morning" scene is the stuff of legend. The four-minute song-and-dance number featuring all three of the film's stars (Kelly, Reynolds, and Donald O'Connor) took 15 hours to get right. By the end, Reynolds' feet were bleeding and she could barely move. Assistants carried her to her dressing room, and doctors put her on bed rest for two days.

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After all of that work, Kelly, who was also the film's co-director, ended up selecting the first take for the finished movie.

The fact that Reynolds could keep pace with her co-stars at all is remarkable. Kelly was 20 years her senior and classically trained in ballet; O'Connor, 27 at the time, was another "triple threat" (actor, singer, and dancer) who had been performing in movies since the age of 12. Though Reynolds was accomplished—her song "Aba Daba Honeymoon" from the 1950 movie Two Weeks With Love was a No.3 hit—she had no training in dance.

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"I had three months to learn what Gene Kelly and Donald O'Connor had been doing for years," Reynolds wrote in her 2013 memoir Unsinkable.

Kelly was a "cruel taskmaster," Reynolds continued. "He came to rehearsals and criticized everything I did and never gave me a word of encouragement."

In a 2012 interview with Newsday, Kelly's widow, Patricia Ward Kelly, said her husband designed routines to suit his partners, whether they were professional dancers or not. "Gene always said, 'You choreograph to the woman,' so he choreographed to Debbie's capabilities....You want your partner to look the best he or she can possibly look. And that was always his intent with Debbie, and she worked like a trouper and became a big star."

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Not only did Kelly disparage Reynolds with harsh remarks, he also subjected her to unwanted sexual advances during their characters' first kiss, according to Unthinkable. Reynolds wrote:

'The camera closed in. Gene took me tightly in his arms . . . and shoved his tongue down my throat."Eeew! What was that?" I screeched, breaking free of his grasp and spitting. I ran around frantic, yelling for some Coca-Cola to cleanse my mouth. It was the early 1950s, and I was an innocent kid who had never been French-kissed. It felt like an assault.I was stunned that this thirty-nine-year-old man would do this to me.'

During dance rehearsals, she once ducked into one of MGM's rehearsal rooms and sat under a piano crying. Another legendary dancer, Fred Astaire, then in his early 50s, was the one to comfort her. "You're not going to die," he told her. "That's what it's like to learn to dance. If you're not sweating, you're not doing it right."

Though it's reported that Reynolds eventually took a perspective of gratitude for everything Kelly had put her through, because it made her a better performer, the exacting director later admitted, "I wasn't very nice to Debbie. I'm surprised she still speaks to me."