All the principal actors underwent several days of grueling army training - except for Matt Damon, who was spared so that the other actors would resent him, and would convey that resentment in their performances.
Tom Hanks was the only one of them who knew it would be a hard and uncompromising experience: “The other guys, I think, were expecting something like camping in the woods, and maybe learning things while sitting around the campfire.”
In the scene in which Tom Hanks’s character tells the rest of the unit what he does for a living back at home, Hanks’ speech was much longer in the original script. But Hanks felt that his character wouldn’t have said so much about himself, and he told director Steven Spielberg so. Spielberg agreed, and the speech was shortened.