According to Judy Garland's biographer, Gerold Frank, The Pirate (1948) was the first film ever in which the studio hired a psychiatrist, paid for out of the film's production budget, to treat the star during shooting and make sure she was mentally healthy enough to perform.
Both Gene Kelly and Judy Garland fought to get The Nicholas Brothers (Fayard Nicholas and Harold Nicholas) included in the movie. They succeeded, but the "Be a Clown" sequence was cut by exhibitors in Memphis and other U.S. cities in the South because it included The Nicholas Brothers, who were black.
While one dance sequence was being rehearsed, Harold Nicholas was just going through the motions, and Gene Kelly accused him of not knowing the routine - so Nicholas danced the whole routine, alone, full-out and flawlessly. Kelly was speechless.