The storm drain car chase scene was shot in a large air duct tunnel below the four main bores of the Fort McHenry Road Tunnel in Baltimore, MD. Fort McHenry permits Interstate 95 to pass under Baltimore's harbor. The air duct is only accessible from the Tunnel's Admin Building by stairs and a small elevator, so the cars in the scene were chopped into several sections, taken three levels below, reassembled and painted. Once filming was complete they were disassembled once again and removed from the duct. The water was hosed in from a nearby sprinkler main.
Gene Hackman turned down the film several times, but was ultimately convinced to sign on after a phone call by director Tony Scott. Will Smith later signed on at a relative post-Independence Day (1996) bargain price because he wanted to work with Hackman.
Reynold's birthday is 9-11. Ironically, the "surveillance society" Hammersly mentions would eventually become the "Patriot Act" passed under the Bush administration post-9/11, only three years after this film is produced.
When Dean sees the article in the paper indicating that he's being investigated by the FBI, he says "They have NO Sullivan protection for this." He's referring to the Supreme Court case New York Times v. Sullivan, which set the standard for defamation cases brought against media companies.
Tom Sizemore played in a scene in True Romance (1993) in which multiple heavily armed groups of men faced each other at gunpoint in a small room with a shoot-out massacre as result. A similar scene appeared in Saving Private Ryan (1998), also featuring Sizemore. This movie contains another such scene, again featuring Sizemore.
Several times the location of a person or vehicle being tracked is given in degrees and minutes of latitude and longitude. Near Washington, D.C., this designates an area of about 1.02 square miles. Not quite as precise as shown.
When Lyle (Hackman) is taking Dean (Smith) into the warehouse "office", one can see that it is a self-made Faraday Cage. One of the few constructs that completely shields those inside from sending/receiving electromagnetic signals.
The trivia items below may give away important plot points.
The picture of a younger Gene Hackman shown in a white shirt and tie, supposedly from his NSA file, is actually taken from The Conversation (1974). Hackman's character Brill, closely resembles his "Conversation" character, Harry Caul. In "The Conversation," Harry Caul, like Brill, is a paranoid surveillance expert who has his workplace in an industrial warehouse. Also, Brill wears the same translucent raincoat worn by Harry Caul in the previous film. In "The Conversation," Harry Caul (like Robert Dean in this film) is pressured to hand over a tape (albiet an audio tape) that has evidence of a murder conspiracy. At the end of "The Conversation," Caul demolishes his apartment when he thinks people who have been observing him might be coming for him. (It's been suggested by more than one film critic that Brill could actually be an older Harry Caul, living under a pseudonym.)