A recent post on The Daily WTF discusses a company where they employ The Receptionist Test. The hiring manager has the receptionist stage a tech support problem and asks the candidate, waiting in the lobby for his or her interview, for help. One guy tries to help with a document that won't print, but doesn't realize the printer is off, and so on.
While tricks like this may not be common, there's a reception test that you run into every time you interview. Every interaction you have with everyone in the company is part of your interview that could have positive or negative effects, and the receptionist is the first candidate. The receptionist comes into contact with hundreds of people every day, and is likely tuned into observing people as they pass through the doors.
Whenever I have an interview, the first thing I do after showing the candidate out is ask the receptionist "What did you think? Any comments?" Usually I'll get something bland like "He seemed nice, I like that car he drove up in." Other times I get more interesting comments like "He took a long time to fill out his application. He spent a lot of time on his phone while he was writing, and didn't seem like he was very interested in the interview." or "It must have been a long trip, 'cause he practically ran in and asked for the bathroom." Those specific comments don't affect much as far perception, but it gives an idea of how you're constantly on display.
How you treat the receptionist speaks volumes about you. Were you polite? Did you say "please" and "thank you"? Or did you just grunt and drool before bothering to put on your Happy Interview Face? The receptionist, and those around you, will know.
It might not even be the receptionist who notices your behavior. Maybe that guy in a suit sitting in the lobby isn't another interview candidate, but the CEO waiting for the CFO to go to lunch. I've even sat in the lobby myself before interviews observing the candidate.
Don't make the mistake of thinking that your scrutiny starts when you step into the interviewer's office. You've been on stage well before that point.