Joseph Gordon-Levitt had prosthetics to make him look more like Bruce Willis in order to play his younger self. He also watched a lot of Bruce Willis films in preparation for the role so that he could impersonate some of his mannerisms.
The scene where Young Joe (Joseph Gordon-Levitt) falls off the fire escape was filmed on the actor's 30th birthday. Gordon-Levitt was left hanging on the stunt wires while the crew sang "Happy Birthday" and wheeled out birthday cake.
The script originally called for Joe to move to Paris when he got older, hence why he tries to learn French. However, Rian Johnson realized they didn't have the money to shoot in Paris. The story was changed so that he goes to Shanghai because the Chinese distributor for the film offered to pay for the crew to film there. Johnson accepted because his best alternative to set the scenes in Paris was to shoot them in New Orleans, which he didn't want to do, and because he felt Shanghai better reflected the future setting of the movie.
According to director Rian Johnson, Noah Segan (Kid Blue) took a number of classes to learn how to spin his GAT gun around his finger. Johnson told Entertainment Weekly that he filmed numerous takes of Segan spinning the 8-lb gun, but ended up using the one take where he accidentally flubbed and nearly dropped it, because Johnson thought it was funny.
A diner was built in the small town of Thibodaux, LA (about an hour west of New Orleans) where all of the diner interior/exterior scenes were shot. Locals saw the diner set and kept asking the film crew on when the new diner was going to open. Director Rian Johnson was later told that the diner set was still standing after Hurricane Isaac moved through in September 2012.
The incredibly large "Gat" pistols are actual production firearms, and not just a prop created for the film. It is a Magnum Research BFR (Big Frame Revolver) chambered in .45-70 Government, a rifle round originally adopted by the U.S. military in 1873. The BFR weighs roughly 4.5 pounds.
Rian Johnson wanted the 1984 Tri-Star logo to open the film, but was unable to obtain it for unknown reasons. Although the 1993 Tri-Star logo precedes US copies of the film, the print version is not featured (due in part to this being produced by Endgame and FilmDistrict). The film's title is the last thing to appear in the end credits.
Bruce Willis fires two modern-looking Personal Defense Weapons in one scene. These are examples of the FN P90, designed and manufactured by FN Herstal in Belgium. Though somewhat futuristic looking, these weapons would have been virtually antiques by the time of the film's setting, having been first produced in 1990.
The trivia items below may give away important plot points.
A recurring motif in the film is clouds, including actual clouds in the sky, cigarette smoke, the dust cloud that Joe uses to defeat Kid Blue, and even creamer poured into black coffee (when Cid kills Jesse, the blood also has a cloud-like appearance). This ties into the "Rainmaker" plot.
In the future, the Rainmaker's henchmen dress in black overcoats and wide-brimmed cowboy hats. During shots of Cid's boyhood room, you see a poster for "Bad Bob" and an action figure on his desk wearing the same black overcoat and wide-brimmed cowboy hat.