I have rarely met a job candidate that handled this situation gracefully. I generally get a "nice to meet you" mumble, mumble, mumble a la Ozzie Osbourne.
These chance meetings are CRUCIAL for a job candidate. Yet, most fail due to lack of planning. Or they believe the job interview is over-and are trying their best to get out the door and have no time to be nice to some random person they just met.
At our company, we also make sure all candidates meet some of their future coworkers. We see how they interact with our current staff. One does NOT need to be a hiring guru to pick out the best candidates by just observing them as they walk around and meet people.
Realize that the interview is not over until you are out of sight. This does not mean you have to develop an oversize personality overnight and become Mr. or Ms. Extrovert, but know that how you interact with others has become more important than ever.
If this type of situation makes you uncomfortable...realize you are not alone. It is entirely legal to practice what you might say, even in casual conversation, in front of your spouse, loved one, or mirror.
G.L. Hoffman is a serial entrepreneur and venture investor/operator/incubator/mentor. Two of his companies have traveled the entire success path from the garage to IPO. Currently, he is chairman of JobDig, and his blog can be found at WhatWouldDadSay.com or at JobDig.com.