According to Ridley Scott (while discussing the filming of Matchstick Men), Sam Rockwell was originally cast in one of the roles and had completed some rehearsals before he became ill/injured and was replaced.
The trivia items below may give away important plot points.
On the face of the wall that the CRT students must climb to finish the first "O" course, there is a Chinese character written in white. This word is pronounced "zhong" and means "end." This word is usually used in conjunction with phrases that signify or imply death. In the same scene, Lt. O'Neil catches a trip wire with her foot that signals death for the entire crew.
An alternate ending was secretly filmed without the knowledge of Disney/Hollywood Pictures Executives. When Ridley Scott first screened the film for execs, he shocked and surprised them with the ending in which Demi Moore dies. Both endings were test screened simultaneously and although the darker ending scored higher with audiences, the happier ending was chosen by executives. Similar to the surprise ending for "Thelma and Louise", the alternate ending for "G.I. Jane" was a dark yet bittersweet one. Jordan (Moore) is killed when she risks her life to save her Commander (Viggo Mortensen) then eulogized on television by the tough Senator (Ann Bancroft). Later, the camera pans through a fresh group of SEAL recruits, among them are three women.