From the Lloyd Rose interview with Stanley Kubrick in The Washington Post, 1987:
But perhaps his luckiest discovery was retired Marine gunnery sergeant Lee Ermey, a Vietnam veteran and former drill instructor who was already employed as Kubrick's technical adviser. After videotaping Ermey insulting and intimidating prospective actor-recruits, an exercise designed to see who would react in interesting ways, Kubrick picked him to play the savagely efficient drill instructor, Gunnery Sergeant Hartman. The D.I.'s dialogue, much of it Ermey's invention, nearly all of it unprintable, forges new frontiers of ear-burning obscenity.