Die Hard was based off a book called "Nothing Lasts Forever" about a 60 year old man protecting his 40 year old daughter from a building overrun by terrorists. When starting to make the movie, they were going to have it star a famous guy known for his songs and experience in the detective film, "The Detective" (1968) but by the time he was called in, he was to old to play this hard-core action hero (even if he was in his 60s in the story). Realizing this, the directors tweaked the story a bit so that they could hire a younger, lesser known actor only known for playing David Addison in the ABC show "Moonlighting" at the time. They were gonna hire Arnold Schwarzenegger, Sylvester Stallone, ect, but they turned it down because a cool, badass character "can't get help from the cops"