It took four months to assemble and combine archive footage of past American presidents from Franklin D. Roosevelt to George W. Bush to create the televised warning from Gabriel. The goal was to create a video representation of a ransom note.
Bruce Willis' stunt double, Larry Rippenkroeger, was seriously injured when he fell 25 feet to the pavement. He suffered broken bones in his face and fractures in both wrists. Production was temporarily shut down. Willis picked up the tab at area hotels for Larry's parents and visited him numerous times at the hospital. Larry also doubles for James Caan in his television series, Las Vegas (2003). Caan came and visited Larry in the hospital and joked around for over an hour. Larry told his parents he was glad when Caan left because he hurt so bad from laughing at his jokes.
The stunt featuring McClane driving a police car into a helicopter took three weeks to rehearse. The shot was accomplished by suspending the helicopter in the air with cables and combining two separate shots: one of the stuntman leaping from the helicopter and one of the car colliding with it. CGI was then used to delete the support cables and add rotor blades.
A water treatment facility near Los Angeles doubled as the film's Woodlawn Social Security Administration building. The facility has miles of underground tunnels, and was also used in Die Hard 2 (1990) when Bruce Willis runs through tunnels under the airport.
According to Bruce Willis and director Len Wiseman in the DVD commentary, the story originally involved McClane's son, Jack. Originally, he was supposed to be the computer hacker John has to deliver to the FBI. Eventually that idea was dropped and the hacker became the Matt Farrell character. It was then decided to bring in his daughter Lucy to keep up the series theme of McClane always having a personal stake in what happens in the story.