The firearms used in the film are real weapons modified to function with blanks, which is the standard protocol for most action films. However, due to McTiernan's request to have vivid "exaggerated realism" in the muzzle flashes, weapons specialist Michael Papac had to specially modify the firearms to discharge his powerful handcrafted blanks which were so deafening that they caused Alan Rickman (along with several other cast members) to flinch. To work around this, McTiernan was forced to cut away from Rickman whenever he fired his gun, but traces of his flinching still made it into the film — if you look at his face when he shoots Takagi you'll notice him wincing.

A happy accident.

The scene where McClane falls down the air vent shaft was a mistake by the stuntman. He was supposed to grab the first vent, but slipped and continued to fall. McTiernan incorporated the shot into the film anyways because it played into the exaggerated realism, splicing it together with a shot where McClane grabs the next vent down as he falls.