The book and screenplay of the movie were written by a man who served in the same Marine unit as my Brother in Viet Nam. He and his fellow Marines were often coptered to the worst and most dangerous outposts and the unit had 50% casualties. My Bro liked the man and was not surprised that he wrote the book. RIP to both of them! Semper Fi!
If you’re referring to Gustav Hasford, the Marine combat correspondent-turned-novelist, he actually lobbied against casting Ermey as GySgt Hartman. From Hasford’s perspective, Ermey was a REMF, despite the fact he served in Vietnam and pulled a tour as a USMC drill instructor (Ermey’s primary MOS was in aviation maintenance, not infantry). Luckily for all involved, director Stanley Kubrick rejected Hasford’s arguments, and cast Ermey in the role.
BTW, Hasford’s choice to play Hartman was (Ret) Marine Corps Captain Dale Dye, who has served as a technical consultant on dozens of military films and appeared as an actor in a number of productions, including Band of Brothers (he played the airborne regiment commander in Band of Brothers). Dye is a capable actor, but Kubrick realized the importance of the Parris Island sequence to the overall film and knew he’d found acting gold in Lee Ermey.
One factor that worked in Ermey’s favor is that Kubrick and Hasford fought constantly over the production. At one point, Kubrick fought to have Hasford’s name removed from the writing credits, arguing that the novelist’s contributions didn’t warrant inclusion on the screen play credit. And he had a point; Ermey created most of his own dialogue for the Parris Island sequence and the combat scenes were largely written by Kubrick and Michael Herr. In the end, Hasford got a screenplay credit (and Academy Award nomination), but he never worked in Hollywood again, and died at the age of 45 in 1993.