Martin Sheen -- "Slovik was given a batch of letters
on the very morning he was shot. The mail had backed up for some months.
So there were many letters that he'd never seen. In fact, he didn't read
them all. He only read a few that morning. He couldn't get through them.
Janet wrote to me a whole batch of letters and asked me not to open
them until the scene was shot, and that's exactly what I did.
One of them was so powerful that it just overwhelmed me. On camera I read it."
Those letters helped him to find the deep emotions he needed to portray the young soldier's final day.
****note: that reading was edited from the movie because it was not historically correct.
(excerpt from Martin Sheen Actor - Activist)