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In the script, John McClane is a "tough-as-nails New York cop" , but John McTiernan and Bruce Willis didn't really have the character sorted out until about halfway through shooting. It was then they figured out this was a guy who didn't like himself very much, but who's doing the best he could. The little moment of McClane banging his head against the door frame after fighting with Holly was a re-shoot done after the character was figured out.
The fictional Nakatomi Plaza is the headquarters of 20th Century Studios, so the studio could use one of its own buildings and didn't have to hold back on stunts and action sequences. While Jeb Stuart was writing the screenplay, he did a tour of the building, and immediately incorporated some of the locations and objects he found there into his script as set pieces (such as the cart that McClane and Karl end up riding during their fight). The company charged itself rent for the use of the then-unfinished building. Some of the middle floors were occupied by legal and administrative departments, so only empty floors were used for filming. Still, the filming of scenes that involved gunfire had to be postponed until after hours because some of the employees from the active floors started to complain about the noise.
The costume department had 17 undershirts in various stages of degradation on hand for Bruce Willis.
In an article for the 30th anniversary of the film, Bonnie Bedelia stated that the first thing she thinks of when someone mentions the movie is Alan Rickman. The two became friends and had lunch together every day while shooting the movie. She expressed how lovely and gentle Rickman was in real life.
The scene where Bruce Willis and Alan Rickman meet up was unrehearsed to create a greater feeling of spontaneity between the two actors.
Bruce Willis received a then-unheard of $5 million fee, which was approved by Fox President Rupert Murdoch.