Jake Gyllenhaal was convinced he had blown his audition, especially after several months passed and he hadn't heard back from Sam Mendes. An impassioned message Gyllenhaal left on Mendes's voicemail swung the decision in his favor.
Sam Rockwell had one scene ("Payback") playing Swoff's uncle Walt, a Marine who served in the Vietnam war and who encourages the young Swoff to join the force but that scene was deleted from the film. It can be found on the DVD and some online sources.
At the time of filming, Peter Sarsgaard was not yet Jake Gyllenhaal's brother-in-law. Sarsgaard first started dating Jake's sister, Maggie Gyllenhaal, in 2002. The couple got engaged in 2006, the same year they had a daughter and were finally married in 2009.
Filmed in the Imperial Valley in Southern California, which features conditions very similar to Iraq. Marines did use one of the local towns, Brawley, for training purposes due to similarities to Iraq.
When Swofford is ordered to clean the latrines, one of the receptacles has the words "Abandon all hope, ye who enter here." This is a famous line from 14th century poet Dante Alighieri's "Inferno", and is the inscription above the gate of Hell as the poet walks through it. The symbolism is that whoever is cleaning the latrine is going through "hell", as it was one of the worst duties for a soldier to have to do.
The desert locations were scouted in the summer months. When filming actually began, it was in the winter after rain, so vegetation had sprung up on what was supposed to be barren land. All of this had to be removed, often digitally.
Iván Fenyö, a Hungarian actor who plays a minor role in this movie, included an easter egg message to his country. When the Marines are setting up their tent, he's seen in the background applying a Hungarian national flag as a decoration.
The PB pills the Marines are issued are pyridostigmine bromide pills. They were designed to increase the effects of the atropine and oxime postexposure antidote that is self-administered to reduce the chances of dying from nerve agents such as soman.
Staff Sgt. Sykes originally had a tattoo of a panther on the back of his shaved head. Jamie Foxx sported it during his award sweeps for Ray (2004). The tattoo was eventually digitally removed in post-production because director Sam Mendes felt it made the character too "hard core."
According to Iván Fenyö, almost 70% of his performance was cut out. Two months before the release of the movie, the director phoned Ivan and told him the studio didn't want most of the lines he had in the movie. According to the actor, some of his cut lines were about his character's observations as an East-European about democracy and the Gulf War.
The trivia items below may give away important plot points.
Jake Gyllenhaal's audition scene was the one where he points a rifle into the face of one of his comrades and has a mini-breakdown. When filming the real scene, Gyllenhaal knocked out one of his own teeth when he turned the gun on himself, as well as hugely upsetting co-star Brian Geraghty, who felt that Gyllenhaal had effectively brutalized him. This led to the later scene in the film where Swofford apologizes to Fergus, a scene that wasn't in the original screenplay. The scene helped smooth over the tense relations between the two actors.