If I remember right, Kubrick originally wanted the whole scene scripted (because again, we are talking about the physical embodiment of "Demands perfection") with a different actor, and Ermey was brought on to ensure authenticity in the script. He pointed out that it is literally impossible to script a Drill Instructor because they are supposed to find weakness in a recruit and attack it so they can eventually be molded into a better soldier, and that requires improv. Kubrick balked at this ('natch), but changed his mind after Ermey submitted a video of himself chewing out a bunch of Marines, as they threw rotten fruit at him, and he didnt repeat himself in a 10 minute rant. Kubrick made Ermey the Drill Instructor, gave him free reign, and the rest is history.

Again, if I am remembering the story right.