I did forget to mention one thing. Because Full Metal Jacket is basically two stories back-to-back reinforcing each other, I'll explain how the plotting method works in both halves. And because there's no pot step for the hero, I'll come out and say that the story's hero is Joker, played by Matthew Modine.
Let's start with the seven key steps I laid out in my last "Truby's Plot" segment: weakness and need. As I said, it really should be called weakness, need, and problem. The problem is the hero's crisis, the dilemma that must be overcome. In Parris Island, Joker's problem is that he has been drafted into the Marine Corps. In Vietnam, he's been deployed to the war.
Next comes the weakness. The weakness is something that the hero lacks, something that keeps him from moving forward. It doesn't necessarily keep him from solving the problem, but it does make it more difficult. Joker's weakness throughout the film is that he is a nonviolent person. This is extremely clear at the start of recruit training, and even later on during the Tet Offensive, he tells a fellow soldier that he's not ready for a fight, even though he earlier voiced the rumor that the Vietnamese would try to attack.
There are two kinds of need, psychological and moral. The moral need isn't required, but it does make for a more interesting story. The psychological need, on the other hand, is required, but not difficult to come up with. Look at the weakness and ask yourself how it hurts only the hero. From Joker's nonviolence, we know that he needs to harden himself in order to become a Marine, and later on in Vietnam, we know that he needs to stop treating the war like a cruise, cracking jokes and bargaining with local prostitutes in Da Nang.
The moral need, on the other hand, is hurting other characters, not just the hero himself. Truby says that there are two ways to find a moral need, and both are used in Full Metal Jacket. The first way is to take a character's strength and turn it into a hindrance for others. In training, Joker's strength is his compassion, but this ends up harming others because he hesitates to resolve a situation violently.
The other, more straightforward method is to look at the psychological need and translate it in a way that hurts others. Joker's sense of humor may be good at diffusing tension, but in a setting like the Vietnam War, it's subtly lowering the guard of his fellow Marines, making them vulnerable to their enemy.