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.
Almost all the main actors were cast because of their close physical resemblance to the real-life soldiers they were portraying.
During the boot camp training, Neal McDonough's weapon went off, and damaged part of his face. After the wound became infected, he had to be taken to a London hospital at ten p.m. Not wanting the press to hear about it, he gave his name as Buck Compton. He also refused Novocaine while the wound was stitched, under the basis that a 1940s soldier wouldn't have had it. He was wearing his costume the whole time. He arrived back at the base at three a.m., just in time for drills.