It is not choreographed. Some of these people do actually have chips on their shoulders. I've seen the frustration they show towards fellow officers while off to the side that they didn't get a chance to 'nail me to the wall.'

I've also heard from a cabbie that crosses the land border often that sometimes the guys in the booth will send someone inside, then come inside and request to deal with that person themselves. This allows for massive amounts of abuse for assholes on power trips. If it has to go through multiple people, there's at least a chance for one officer to say, "WTF" if another is out-of-line.

Some people are super-nice, and others are complete assholes. This is not by design. If it was all choreographed, then the experience would be very much more homogeneous.

That said, there is a difference between a 'stern line of questioning,' and making an ass of yourself. Conversations like:

  Officer: Question1?
  Me: Answer, because...
  Officer: Stop trying to qualify your answers! I'm asking
           the questions here asshole. Just answer *exactly* what
           I ask you, are you stupid or something?

Are the office being an asshole. I've only had these conversations happen a handful of times, over a ton of land-border crossings, but they do happen. Saying that it's all 'choreographed' just gives these guys a blank cheque to do whatever the hell they feel like and get away with it. Having a bad day? Just take it out on people crossing the border! It's not like they have any rights or anything once they get in line! No one will care if you abuse them! (This is obviously a recipe for only good things!)

Indeed, I had gotten back from a fortnight holidaying around Croatia and flew into Gatwick in Britain on an Irish passport (with a New Zealand accent). I got stopped at the EU entry and demanded I explain because my passport was fake.

Later I found out its a common ploy they use to illicit an emotional response and they watch for tell tale signs that I would give myself away if I was lying.

Unfortunately for the lady the holiday was roughly a week of heavy drinking on yatchs and partying all night, so I was absolutely exhausted. So I let out a rather unrepeatable hail of abuse at how stupid she was and the poor woman didn't look up to meet my angry gaze and just stamped my passport... and I walked off nervously, full of guilt, expecting a security guard to escort me off for questioning or something but no one did thank god.

So they have all sorts of things up there sleeves to try and read your intentions from the get go, I've never had any abuse myself though, but then I've never travelled into America before either.

True, but, it seems some guards tend to take the role more seriously than others.

Case in point, my co-workers and I would always have difficulty going through US customs (which, is located in our Canadian airport) when questioned by one specific agent.

If we go through US customs and get any of the other agents, 9 of 10 times there would be no issue at all.