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 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.
The character of Salt is named for the Strategic Arms Limitation Talks (S.A.L.T.). The talks, in 1969 and 1979, were between USA and Soviet Union during the Cold War.
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.
The screenplay for this film was featured in the 2007 Blacklist; a list of the "most liked" unmade scripts of the year.
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.
There's a small pennant with a Russian tricolor hanging behind the LCD screen at SALT's working desk. The Russian lettering on it says "To the participant of the war campaign in Chechnya".
Dutch-Palestinian director Hany Abu-Assad was attached to direct the film. But the green-light committee thought he was too unknown to make such a big budgeted project.
During the car-chase scene in New York, you can see the sign for Silvercup Studios where part of this movie was filmed.
Terry George, Michael Mann and Peter Berg all came close to directing at one point or another.
The only non-Best Picture Oscar nominee that year to be nominated for Best Sound Mixing.