"It got so complicated because some the letters were written by children, some of them had moved, some of them were written by women who got married and changed their name," Fitzpatrick said. "It was incredible detective work to find these people."
Of the roughly 250 letters Fitzpatrick selected, her team found all but 20 of the letter writers or their relatives. Only five people declined to give their permission.
When Couturie approached Fitzpatrick in 2010 about turning her book into a film, they worked together to select a group of letters that were eloquent but also could be used to visually illustrate Kennedy's presidency. In order to use the letters in the film, the team had to re-ask permission from the original letter writers or their living family members.
One condolence letter that struck a chord with both Couturie and Fitzpatrick was the "I am but a humble postman" letter, written by Henry Gonzales of El Paso, Texas, in 1963. In it, Gonzales, a Mexican immigrant, identifies with the affluent Kennedys by drawing parallels between their two families.
"It really captured a central theme, a strong identification Americans had with Kennedy... despite the obvious differences in their background," Fitzpatrick said.
To tell his story in the documentary, Couturie said he spoke with Gonzales' son, who gave him some of his family's old home movies.
"And so I found these scenes that perfectly mirror what you saw on the Kennedy's home movies," Couturie. "And that was one of those things that was just pure luck, that the guy who wrote that letter had filmed all these home movies and his son had kept them tight all these years."
At the heart of the documentary is what Couturie called the "incredible irony" that Kennedy was forced to take a stand on civil rights after Mississippi Gov. George Wallace's hate-fueled denouement of desegregation, but that Congress couldn't pass the civil rights bill until after Kennedy was assassinated.
The George Davis letter, read by Oscar-winning actor Chris Cooper in the documentary, addressed those issues. Born in Montgomery, Ala., Davis wrote to Jackie Kennedy in 1963 that he had procrastinated in writing the president a letter of support, and in that way, had failed him.
"Multiply my procrastination by that of thousands in the Southland who much have sympathized with his efforts, and our neglect takes on the proportions of tragedy -- especially now," Davis wrote.
"The George Davis letter punctures that Deep South, anti-segregation sentiment and so I love that letter," Couturie said.
Couturie chose a number of Hollywood A-listers to read letters for the film, including Anne Hathaway, Jessica Chastain, Allison Janney, Viola Davis, Michelle Williams, Channing Tatum, Mark Ruffalo and Cooper, among others.
The director said he wanted to use professional actors to read the letters, not just to help boost the film, but also because they understood how to read with appropriate emphasis -- some of the letters were edited down for time, but were otherwise read as written.