Angelina Jolie, 39, revealed to Australia's TV Week magazine that she and Pitt, 50, sent handwritten letters to each other while they were filming on opposite sides of the world.
The actress and human rights activist was directing Unbroken, a biopic about World War II hero Louis Zamperini, in Australia, while Pitt was in London filming Fury, about a tank crew fighting the Nazis.
"He was supportive from a distance, and it was quite romantic in a way," she says.
"We decided to be of that time, when we could imagine he was in the European theater and I was in the Pacific theater, and we wrote handwritten letters to each other that were very connecting for us, thinking of the people that were separated for months, if not years, at a time back then."
Jolie also told the magazine she was able to show a rough cut of Unbroken to Zamperini shortly before he died at age 97 on July 2. "I brought him the film on my laptop in the hospital, and it was amazing seeing someone at the end of their life watching their life unfold again, at the same time their body was shutting down," she says.
"He wanted me to make the movie to show something hopeful about the strength of the human spirit that can pull us through. He reminded me to have my surgery in the year that I did, and he reminded me to appreciate every day of my life."
Jolie underwent a preventive double mastectomy in 2013, while she was preparing to direct Unbroken.
Next up for Jolie and Pitt: making a movie together.
Universal Pictures recently announced that the engaged couple will work on an "intimate, character-driven drama," By the Sea. Jolie and Pitt will both star in the movie, with Jolie writing and directing.
Jolie's Unbroken will premiere Christmas Day, while Pitt's Fury is set for November.