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.
Joe Liebgott (played by Ross McCall) is portrayed as Jewish in the miniseries, and based on his name, appearance, and utter hatred of the Germans, his fellow members of E Company all assumed that he was Jewish, but the real Liebgott was a Roman Catholic, the son of Austrian immigrants; according to his son, his especially virulent hatred of the Nazis stemmed largely from his belief that they gave Germanic people in general a bad name. He was apparently aware that his fellow troopers thought he was Jewish, but never bothered to correct them, as he found it amusing.