"It was very scary to take on somebody that people know so well - the way she looked, the way she spoke, the way she moved," Portman admitted to ET's Nischelle Turner. "You have to overcome that hurdle before you can even get people to sign on to emotionally empathize with you."
In the film, the 35-year-old Oscar winner plays the celebrated first lady during her days in the White House and following John F. Kennedy's 1963 assassination. Portman praised the film for not idolizing its subject.
"It doesn't try to tell you what to think about her," she explained. "It allows her to be a human being and you see all of these different aspects of her, and it also leaves a lot of mystery."
To prepare for the role, Portman watched many of Jackie's interviews, as well as a tour that she gave of the White House in a TV special that aired in 1962. During that tour, Portman thinks the first lady might have had some sort of foreshadowing about her husband's fate.
"She talks about how first ladies have to leave the White House," Portman said. "Always have to move out. And she talks about Abraham Lincoln a lot. Watching it in retrospect, you get the sense that she almost had this instinct that something was coming."
The actress, who is pregnant with her and husband Benjamin Millepied's second child, might be experiencing some foreshadowing of her own, but in a good way. The last time she was pregnant, Portman won an Academy Award for her performance in Black Swan. Now, she is already getting Oscar buzz for her work in the upcoming film.
"They're good luck charms in life," said Portman, who gave birth to 5-year-old son Aleph on June 14, 2011, of her children. "They're the best things."