In the often meticulously controlled world of Stanley Kubrick films, it’s hard to imagine anything making it into the final cut without first going through rounds of rewrites, rehearsals, and retakes.

But in Full Metal Jacket, one of the film’s most unforgettable sequences - the boot camp barrage of insults hurled by Gunnery Sergeant Hartman - came not from a polished script, but from a real-life drill instructor.

A Vietnam War veteran and former Marine drill instructor at the real-life Marine Corps Recruit Depot in San Diego, R. Lee Ermey wasn’t even cast in the role to begin with.

Before stepping in front of the camera, Ermey was hired as a technical adviser, brought on to coach actors on how to convincingly portray life in Marine Corps boot camp.

Kubrick, known for his methodical directing style and tendency to exert creative control over every detail, had originally cast actor Tim Colceri in the role of Gunnery Sergeant Hartman.

But after watching Ermey perform an off-the-cuff demonstration of how a real drill instructor would address new recruits, Kubrick was so impressed that he rethought the role entirely.

To land the part, Ermey filmed a 15-minute tape of himself yelling insults while being pelted with tennis balls and oranges off-camera - just to prove he could keep his composure and stay in character.

Kubrick was reportedly so taken by the performance that he rewrote the structure of the film’s first half around Ermey’s delivery.

Almost every line said by Hartman in the film’s boot camp sequences - the put-downs, the threats, the dark humour - was improvised or ad-libbed. The actor prepared his own lines based on years of experience in real Marine Corps training and delivered them with brutal conviction.

Full Metal Jacket, released in 1987, was Kubrick’s penultimate film and stands among his most divisive. Some critics found its two-part structure - the first half at boot camp, the second in war-torn Vietnam - jarring, but nearly all agreed that the opening act was unforgettable, largely because of Ermey.

The raw power of his performance earned him a nomination for a Golden Globe Award as Best Supporting Actor. He went on to appear in about 60 films after that - mostly in roles involving military positions and authority figures - inlcuding a voice role in the Toy Story franchise. He passed away from complicatons related to pneumonia in 2018.