1. Die Hard is a sequel, in a way, to the Frank Sinatra movie The Detective.
The Detective was based on the novel by Roderick Thorp (a name you might recognize). In that movie, Frank Sinatra played Joe Leland, a NYC detective.
Roderick Thorp went on to write a sequel called Nothing Lasts Forever, again featuring Joe Leland. That sequel involved terrorists taking over an office building, with Leland caught in the crossfire.
2. The lead role in Die Hard was initially offered to Frank Sinatra.
Sinatra was 70 years old in 1985, when Die Hard was being put together as a concept. The producers didn't really want Sinatra to star in the action movie -- but it was legally required that they do so, because Sinatra had some kind of legal option on the sequel.
They offered him the role, hoping he'd pass. He did. He decided he was too old to play the role again.
And thus, "John Leland" became "John McClane," so as not to confuse people into thinking this was in fact a direct sequel to 1968's The Detective.
3. The John McClane role was offered to just about every warm body in Hollywood.
After Sinatra passed, the producers offered the lead role to all the people you'd guess they'd offer an action movie lead in the mid-80s to: Arnold Schwarzenegger, Sylvester Stallone, Clint Eastwood, Mel Gibson, James Caan, Richard Gere, Don Johnson, and others.
Pretty much the role was offered to anyone not named "Bruce Willis."
They were all either uninterested or unavailable. I think they tried to sell Schwarzenegger on the idea that the story would make a good sequel for his "John Matrix" character in Commando.
Getting desperate, they offered it to television actor Bruce Willis, who played a detective on Moonlighting, but as a glib comedic figure, not as a badass.
The studio was very skeptical that Bruce Willis, hot off the non-success of the romantic comedy Blind Date, could carry a movie at all, let alone an action movie.
The studio eventually put out a marketing blitz to sell people on the idea that Bruce Willis could even be an action star.
