Moira Shearer was a gorgeous dancer who didn't get her due. Before her film debut, she was soloist at Sadler's Wells Ballet which became The Royal Ballet. Margot Fonteyn was a huge ballet star but she was aging and many younger dancers hoped that her retirement in a few short years or so would allow them to move up in rank. Shearer was one of those dancers.
From what I understand she kept refusing Powell and Pressburger's offers to cast her as the lead in their film The Red Shoes. She just wanted to concentrate on her dancing. Her constant refusals just made them want her more. It was Ninette de Valois who urged her to take the part because she was sick of Powell stalking her ballet company.
Shearer took the role, it became a huge hit turning her into an overnight movie star sensation and, sadly, derailing her ballet career. I believe she suffered in the company due to jealousy. Not only that, Margot Fonteyn proved to have the strength of 10 ballerinas. She was nowhere near retiring and when she considered it, Rudolf Nureyev appeared and asked her to be his partner. Ultimately, Shearer retired before Fonteyn.
At one point in her career, she had the opportunity to dance for Balanchine. He was so impressed with her that he asked her to come to NY and dance for him. She sadly refused him. Maybe she still hoped Fonteyn would slow down her dance schedule. Perhaps she was shy of Balanchine. He already had a reputation as a womanizer. But judging by this film clip, she could have been one of NYCB's first legends. A truly poignant what if.