Also, Bruce Willis may have signed up before reading the script, often with franchises/series they sign them up for prospective sequels before/whilst writing them. All the main cast for The Hunger Games have signed up for 3 more films, and they haven't even finished the script for them yet. (Though because they're based on books, the cast presumably have some idea where it's going to go...
Someone like Jennifer Lawrence/Daniel Radcliffe will have to sign a 2 or 3 film series deal as part of a longer contract. This is to assure that the series continues every couple of years and hits the same demographic.
Bruce Willis is way beyond that as an actor and a hollywood commodity, he had no commitments to this Die Hard before being presented with the script and a nice fat cheque.