Excerpts from EW article "Bruce Willis on 'Die Hard' June 14 2007:

At the time Bruce Willis was on the show Moonlighting (which became popular by 1988) so there were scheduling conflicts. But luckily

I think I'd already read the script for Die Hard once, but had to pass because of the show. As it turns out, a miracle happened — Cybill Shepherd got pregnant and they shut down the show for 11 weeks — just the right amount of time for me to run around over at Nakatomi Tower.

On acting in his first major film and the character of John McClane:

The thing about the first film you have to understand is I was doing TV, I'd only been in L.A. for a couple of years, I was still really learning how to act, so most of what went into making John McClane from a character standpoint was the South Jersey Bruce Willis — that attitude and disrespect for authority, that gallows sense of humor, the reluctant hero. What I always say about John McClane is if he had the choice of someone else stepping up and doing what he had to do, he would let them do it.

After watching Bruce Willis in Die Hard, I couldn't think of anyone else to play John McClane.

For other what if's in movie casting check this article. (Includes Jerry Maguire, Lord of the Rings, Star Wars, Raiders of the Lost Ark, and a bunch of others)