When Aristotle Onassis, the renowned Greek shipping magnate, crossed paths with Maria Callas, the extraordinary opera diva, in 1957, their lives took an unexpected turn. Both were already married to others, yet this did not deter their hearts from falling in love. Their affair would endure until Onassis' passing in 1975, leaving a lasting mark on history.

The Intimate Details

Kiki Feroudi Moutsatsos, Onassis' long-time personal secretary, shares insights into their relationship. "They couldn't exist without each other," she tells PEOPLE. Once their affair began, it was a continuous affair, never ceasing. Even after Onassis divorced his first wife and married Jacqueline Kennedy in 1968, their connection remained strong.As Moutsatsos recalls, "Onassis was safeguarding his affair. He was protecting Jackie, but Jackie was highly intelligent. Everyone in the house was secretly discussing that Onassis visited Maria every night. She knew and even discussed it with Onassis' sister, Artemis. She was complaining, but Artemis advised her to ignore it. After all, it wasn't something that could be easily changed."The story of their tumultuous affair is vividly depicted in the new movie Maria, directed by Pablo Larraín and starring Angelina Jolie as the famous soprano. Aristotle Onassis and Maria Callas on June 24, 1959, captured a moment that would become part of history.Evening Standard/Hulton Archive/Getty.Few were as close to the truth as Moutsatsos. She began working for Onassis when she was just 17 and later wrote the 1998 memoir The Onassis Women, which she hopes to republish. "They frequently met in Paris," she recalls. "Their houses were in close proximity. Onassis had an apartment on Avenue Foch, and Callas lived at 36 Avenue George Mandel, both in the posh 16th arrondissement."As she recounts, "When I was making calls every morning, whether in Athens or Paris, and communicating with the personnel, they would make jokes. They would tell me Mr. Onassis was extremely tired. When I asked why, they would say again that Maria was visiting."Maria Callas and Aristotle Onassis. Getty Images."We knew every moment of what they were doing," says Moutsatsos about all the women in Onassis' life. "Because I was handling the arrangements from my office and could make all the details of their trips. Christina (Onassis' daughter) and Jackie would call me to inform me of their schedules in Paris and the United States."However, Callas was not always content with her lover. As Moutsatsos recounts, "When she was on the boat (Onassis' yacht, the Christina), she would read the newspapers and be exposed to all the rumors and scandals about Aristo. They would have arguments."Jackie Kennedy and Aristotle Onassis. Bettmann Archive."He had a penchant for being surrounded by beautiful women, and Maria was very jealous. She was highly nervous. When articles appeared in newspapers or magazines, she couldn't bear it. She was constantly fighting with him."On the other hand, Jackie rarely showed her emotions. Their marriage faced difficulties in later years as they spent time apart, yet Onassis continued to see Callas.The opera singer never fully recovered after his death on March 15, 1975. Moutsatsos recalls, "Maria was in a state of desperation. She didn't want to live anymore."As for Angelina Jolie's portrayal of the dramatic opera star, she has said, "I hope what audiences discover is that a great deal of research was done to understand what she was really like - the human being behind the voice and the image. Maybe not 'behind the voice' because the voice is the essence of the woman," she told The Hollywood Reporter.Angelina Jolie and Pablo Larrain on the set of "Maria". Pax Jolie-Pitt/Netflix.Never miss a story - sign up for PEOPLE's free daily newsletter to stay updated on the best of what PEOPLE has to offer, from celebrity news to compelling human interest stories."But I think even if I had read biographies about her, I [wouldn't have understood her] until I stepped into her shoes and felt her. I hope more people discover her and opera and attend opera performances and listen to opera."Maria is now showing in select theaters and will be streaming on Netflix on December 11.