Prior to production, the various directors were told that the actors were in contact with the veterans they were playing. If the actors said that the veterans disapproved or disagreed with something in the script, it would have to be changed. Many of the actors frequently got themselves taken out of certain scenes after the respective veterans said they weren't there for the event in question.
Donald Malarkey is seen meeting an American-born German POW who had lived in Oregon, Malarkey's home state, but whose family had returned to Germany before the war. That incident actually happened, but with one crucial difference. In the film, Malarkey hadn't known the man back in Oregon. In real life, the two had actually worked across the street from each other for years.
Tom Hanks, Steven Spielberg, and Stephen Ambrose showed each of the scripts around to real-life soldiers of Easy Company to guarantee authenticity.
Scott Grimes joked that he wasn't acting during Malarkey's sad shower scene, citing that as the point where he really started missing home.
Almost all the main actors were cast because of their close physical resemblance to the real-life soldiers they were portraying.