The martial art Bonham teaches Hallam, and which they employ against each other, is called Sayoc Kali. It is a variation of Eskrima, the Filipino martial arts system dedicated to weapons. Sayoc Kali deals exclusively with knives in extremely close-quarter combat.
The knife used by Hallam (Benicio Del Toro) throughout most of the movie is a Beck Wilderness Survival Knife, designed by Tom Brown Jr. (a tracker/survival expert who was a technical advisor on this feature) and constructed by David Beck.
Tommy Lee Jones had a good working relationship with William Friedkin and agreed to sign on to the film after the reading the script. Reportedly, he also received almost $20 million for the role - his biggest payday at the time.
When Hallam tells Bonham the men he killed weren't hunters, "they were sweepers", he means he thinks they were sent to kill ("sweep") him. William Friedkin says on the DVD audio commentary that it's left ambiguous whether that is true.
In the scene where Agent Hewitt hands Van Zandt a letter demanding Hallams release. The letter is signed by Attorney General Scott A. Anderson. Scott M. Anderson was the assistant property master for the film.
The Metro Area Express (MAX) light rail train used on the Hawthorne Bridge scene was actually two articulated buses joined together at the rear ends, with the frontal portion of the "train" to look like the actual older Bombadier MAX trains.
David and Peter Griffiths screenplay was a hot item in Hollywood. Optioned by producer James Jacks, the film did not go into production until Paramount agreed to finance the picture as a directing vehicle for William Friedkin.