MCCLANE: It's okay, I'm a cop. Trust me. I've been doing this for eleven years.
One of McClane's first lines in the movie tells us his occupation. And a few minutes later, we learn he's NYPD. As his later conversation with Argyle suggests, McClane's very loyal to the force.
MCCLANE: Why the f*** didn't you stop 'em, John? 'Cause then you'd be dead, too, asshole. Think, goddamn it! Think!
Poor McClane's torn between his desire to help out Takagi and his need to survive. If he had stepped in to save Takagi, he'd likely be dead himself. Or, at the very least, he'd be a hostage—and then they'd all be nowhere. But still, it can't have felt very dutiful to sit back and watch his wife's boss get blown away.
ELLIS: John? Aw, Christ. He can f*** this whole thing up. What does he think he's doing?
HOLLY: His job.
ELLIS: Bulls***. His job is 3,000 miles away.
Ah, but a cop is never really off duty, is he Ellis? Sure, John McClane may be NYPD, but he's more than just a jurisdiction.
MCCLANE: I read you pal. You the guy in the car?
POWELL: What's left of him. Can you identify yourself?
MCCLANE: Not now. Maybe later. Listen fast. This is a party line, and the neighbor's got itchy trigger fingers. All right, here's the deal. You got 30 or so hostages on the 30th floor. The leader, his name's Hans. […] They got a freakin' arsenal up here. They got missiles, automatic weapons, and enough plastic explosives to orbit Arnold Schwarzenegger. They're down to nine now, countin' the skydiver you met. These guys are mostly European, judging by their clothing labels and cigarettes. They're well-financed and very slick.
POWELL: Well, now how do you know that?
MCCLANE: I've seen enough phony I.D.s in my time to recognize that the ones they got must've cost a fortune. Add all that up, I don't know what the f*** it means, but you got some badass perpetrators, and they're here to stay.
POWELL: I hear you partner. L.A.'s finest are on it, so light 'em if you've got 'em.
MCCLANE: Way ahead of you, partner.
Just a quick conversation, and the two officers are already calling each other partner. McClane may be a fish out of water in L.A., but it's clear he knows his way around a hostage situation.
HOLLY: I have a request.
GRUBER: What idiot put you in charge?
HOLLY: You did, when you murdered my boss. Now everybody's looking to me. Personally, I'd pass on the job. I don't enjoy being this close to you.
Holly's got duties, too. She's a pretty good caretaker of the hostage crew—that is, until Hans kidnaps her and sends everyone else to the roof. And anyone who mixes duty with sass wins some major points in our book.
CHIEF ROBINSON: Now you listen to me, mister. I don't know who in the hell you think you are or what you're doing, but you just destroyed a building. Now we do not want your help. Is that clear? We don't want your help. I've got a hundred people down here, and they're covered with glass.
MCCLANE: Glass? Who gives a s*** about glass? Who the f*** is this?
CHIEF ROBINSON: This is Deputy Chief of Police Dwayne T. Robinson, and I am in charge of this situation.
Deputy Chief of Police Dwayne T. Robinson thinks he's doing his duty protecting the good people of Los Angeles from the heinous nightmare that is broken glass. Of course, what he's really doing is making a fool of himself and a mockery of the LAPD.
MCCLANE: Hans, this asshole is not my friend! I just met him tonight. I don't know him! Jesus Christ, Ellis, these people are gonna kill you! Tell them you don't know me!
Here, McClane does his level best to save Ellis's life, but no matter how hard he tries to do his duty, he just can't succeed. See, the real problem here is that Ellis interfered with McClane's job. If he had just sat tight, like Holly told him to, he wouldn't be in this mess.
POWELL: No sir. You couldn't drag me away.
We bet Dwayne Robinson's wishing he had some wild horses right about now.