Prop Psychology

As I was standing in line today waiting to get my Five Guys burger (which are little meat morcels of divinity), I noticed something that never really occurred to me- objects can steer our perception of the world, and the outcome of events.  Consider the following situation:

You're in line at the grocery store with your friend and there's this hot girl/guy working the register.  They ask how you're doing, scanning your groceries, and that's generally the end of the conversation (maybe ask if you found everything okay).  You two are talking about something that other people would most likely have an opinion on, like how is Dancing with the Stars still on television.  How inclined are you to strike up much more conversation past this point?  It obviously depends on your personality, but compare your answer to this:

Your waitress comes over to ask what your having for dinner while you're with your friends.  You're in the middle of some light-hearted debate- I don't know, maybe the age old debate of can communism truly exist in any civilization (okay, maybe something she'd know about like what's more manly, whiskey or beer)- how much more likely are you to actually talk to her now? 

Now whether you realize it or not, there are multiple barriers in the first scenario that typically turn most people off from carrying on more of a conversation- those barriers are actually physical barriers.  What seems interesting is that once you notice these things are here, it almost makes conversation beyond the basic greeting feel forced and uncomfortable.  The girl behind the cash register will most likely clam up a bit more than the girl standing right by your table at the restaurant.  What's interesting is that psychology dictates that when a person is standing over you that it's demonstrating a sense of dominance, and should theoretically make you feel less likely to talk to that person.  But compare that to the grocery store where everyone is standing (an equal playing field) and all of a sudden that counter/conveyer belt/cash register becomes a sound-dampening window with two people struggling to communicate. 

The other thing that crossed my mind is that when the first situation would happen to me, I'd go "Oh, well she's at work so she's thrown off guard by interaction beyond the pleasantries."  But then that wouldn't explain the waitress who seems to always have an opinion about everything.  Then the thought occurred to me, "maybe it's the type of people that apply to these different jobs".  But both occupations are virtually the same- little experience required, no education required, interaction with customers.  Sure waitresses get hired on looks, but girls with those same good looks can always be found as a cashier at somewhere and remain hushed when you're around them.

Lastly, perhaps the most likely reason, is monetary gain.  Waitresses get tipped for being personable and bubbly.  Therefore, they see a direct correlation between bubbly and hella cash.  This could be true, but I can think of several occasions of friendly conversation with a girl in Home Depot or Best Buy out on the floor. 

So my question basically is, are the "props" involved really causing this crazy social phenomenon, or is there some unwritten rule somewhere that these occupations can "talk to customers" but not "talk to customers"?