The movie was originally written with Salt as a male (Edwin A. Salt), and Tom Cruise was approached to play him. Ultimately, he backed out and the script was rewritten with a female lead and Angelina Jolie Pitt was cast.
The compound that Salt makes in the interrogation room, just before the tactical team attempts to gas her while she's still in the CIA office building, is a mixture of ammonia and iodine. This makes ammonium triiodide, a very sensitive contact explosive, but only when the brown precipitate is completely dry. The mixture forms a brown paste that dries into crystalline form. Any pressure on the crystals will trigger them to explode, even a breath of air can set them off.
When the police first burst into Salt's apartment, Peabody stops and examines a matryoshka doll he finds on a shelf. The doll was meant to be a symbolic foreshadowing of the rest of the movie - the unraveling mystery of who Salt is and what her part might be in the assassination conspiracy.
The original design of the bunker (the Presidential Emergency Operations Center (PEOC)) where the last part of the movie is set, is highly classified. So production designer Scott Chambliss based his designs on the old German government bunker in Ahrweiler, near Bonn.
It's never stated explicitly beyond "present day," but the main action in this film takes place "in the near future" after the movie's release (summer of 2010). Evidence: In a scene shortly before the vice president's funeral, a long shot of a TV newscast shows his date of death as 2011.
The trivia items below may give away important plot points.
Liev Schreiber accidentally gave away the twist ending while he was promoting the film on The Daily Show with Trevor Noah (1996). During the interview Jon Stewart referred to a "hot Russian spy" to which Schrieber replied, "Are you talking about me or Angelina?" Realizing his slip Schrieber quickly added, "She's the hot Russian spy. I'm the good- I'm her boss. Sorry. Whoa."
When Peabody sees Salt's CIA history on his hand held device, it foreshadows many of the skills she will need throughout the film. He reads: Salt, Evelyn Russian External Operations CIA - Central Asia Division Position: Covert Operations Division: Russia Security Clearance: Level 1 INTERNAL RECORD Current Cover: Vice President of Product & Development, Rink Petroleum Deniability: No agency cover. Capture in Foreign Territory (Hostile or Friendly) Carries Government/CIA Disavowal By United States Special Skills: Close Quarter Combat; Applied Explosive Technique; Rapid Assault; Counter Surveillance; Driving Technique; Covert Communications; Military Free Fall Parachuting; Edged Weapons; Precise Application of Lethal Force; Unarmed Combat; Threat Elimination Commendations: Received the Intelligence Medal for an Act or Achievement significantly Above Normal Duties, Resulting in an Important Contribution to the Mission of the Agency. Recipient of the Featherstone Grant for Outstanding Post Graduate Work in Russian History and Literature. Nationally rated chess player. Black belt in Taekwondo at age 16. Parents killed in car accident, December 24, 1988. After release from North Korea, married German National Michael Krause. Guardian died March 23, 2002. The "Distinguished Intelligence Medal" as worded is real. There is no public record of a CIA-given "Featherstone Grant".