In an interview with Elle, the actress - who portrays the famous First Lady in the upcoming History channel mini-series The Kennedys - revealed that she put all her effort into the role as she was aware of how important it was to tell the family's story accurately.
"[The Kennedys] were like movie stars. There was such a professionalism to their public image," she said.
"I worked as hard as I could, because I loved her. On set there was this feeling that we had to create something very special, because these people were very special."
Holmes went on to say that the role allowed her to get to know more about and appreciate Kennedy's struggles, adding: "I didn't realise that she had lost a child in August and then [John F. Kennedy] was assassinated in November.
"I couldn’t believe it. She had so much strength... I loved her love of family, how important it was for her to be a good mother."