In the first Die Hard (1988), John McClane only had a few scripted one-liners. However, Bruce Willis ad-libbed many one-liners, and audiences liked them. So much so that in this sequel (and the next one), more gags were added, and Willis was told he could ad-lib as much as he saw fit.
The MP5K used by Ken Baldwin jammed repeatedly firing blanks while filming this scene, so his angry outbursts (including the instance where he curses at the gun) were unscripted and genuine expressions of frustration.
At 12 mins.) The airport employee who opens the security door for McClane had no prior acting experience and was actually homeless when the film's casting director spotted him and offered him the gig.