Cast the Expert: R. Lee Ermey was originally just a technician on the film, there to make a couple of films to advise the actor playing the Drill Sergeant Nasty on how it should be done. Then Kubrick saw his tapes... said tapes were of Ermey performing his Drill Sergeant Nasty speeches while being constantly pelted with rotten oranges for fifteen minutes. He never stopped, flubbed, or repeated himself throughout the whole thing.
Completely Different Title: While a few countries went for variations for "metal jacket", in South America (plus Portugal) went for the poster, translating Joker's "Born To Kill". Central America went for yet another scene, Cara de guerra, "War face".
Dyeing for Your Art: Vincent D'Onofrio gained so much weight for his role as Pvt. Gomer Pyle, that he tore ligaments in his knees during the obstacle course scene. He surpassed Robert De Niro's record (for Raging Bull) for most weight gained for a role, over 70 pounds. He would gain weight again for Wilson Fisk in Daredevil, a version of Marvel's Kingpin that has some traits of Gomer Pyle.
Enforced Method Acting: To make the recruits' reactions to Gunnery Sgt. Hartman as realistic as possible, Stanley Kubrick made sure that R. Lee Ermey did not socialize with the actors playing recruits. They also didn't get to meet him prior to filming.
Fake American: The gung-ho Colonel who asks Joker why he wears a helmet that says "Born to Kill" and a peace sign is played by Canadian actor Bruce Boa.
Fake Nationality: The actress who played the Vietnamese hooker is actually of Chinese and French descent. Arguably averted — a mixed-race ethnic Chinese and French bastard could easily have been found in 1960s Vietnam, and would have been a likely candidate for the sex trade in that time and place.
Many of R. Lee Ermey's lines as Hartman were ad-libbed. He was one of the few actors allowed to go Off the Rails in Stanley Kubrick's films, who is known for his meticulous control of every part of the story. When filming the scene where GySgt Hartman is berating new recruits, the "reach-around" comment was tossed in by Ermey, and not originally in the script. Kubrick stopped the filming to ask what that meant. After it was explained, he laughed, and decided to include it in the final film.
Ermey is also a notable subversion. Kubrick disliked improvisation as it usually played hell with his shooting schedules. Ermey, in a major role for the first time and eager to impress, spent so much time with the dialogue coach that he was able to finish his scripted material in very few takes, allowing him the time necessary to throw in his own flourishes —all added to the shooting script and delivered verbatim— with Kubrick's blessing.
Val Kilmer auditioned for Pvt. Joker. According to Matthew Modine, Kilmer confronted Modine in a restaurant and challenged Modine to a fight because he believed that Modine had stolen the part from him. But Modine was not even aware of the film at the time. Modine later sent Kubrick footage from Vision Quest and won the part.