The pair, who have known each other for about 20 years, previously co-starred in the 2004 movie Garden State.
"The transformation was so smooth that it didn't look like anything," Sarsgaard told reporters in New York about the way Portman channeled the late first lady.
"She was very relaxed with it, which is the only way to be. It didn't seem to require a Herculean effort," Sarsgaard explained. "She slipped right into it. She would joke sometimes about how her accent was bad or she'll fix it later in ADR or something like that and I'd hear her doing it and I would be like, 'No, it's absolutely perfect.' To me, if she had gone any further with it, it would be very difficult to... because [the real Jackie Kennedy] had a very intense way of talking, very pre-meditated, too. It always sounded like she had clearly thought out the idea and maybe said it a number of times. The off-the-cuff [interview] thing, that's a little bit more in the Billy Crudup scenes [of the film,] is a part of her that I'd never seen. To me, that's the most imagined part."
The movie dramatizes the 1963 assassination of Jackie's husband, U.S. President John F. Kennedy, and the hectic days that followed. Sarsgaard plays Bobby Kennedy, the president's brother and attorney general who was himself murdered during his run for president in 1968.
Jackie is in theaters now.