Despite being an introvert, yes, I would reciprocate in conversing with a stranger in a crowded elevator. It would actually be much easier for me to converse with a stranger with more people around because it would be less intimate and would be less likely to go in "creepy" directions. If the conversation (or banter) is upbeat and/or humorous, I would hope that it might lift everyone's spirits (and not be an annoyance to anyone).

It would depend on my circumstances for the day. I would at the very least reply courteously, but perhaps a bit curtly if I was on my way to a meeting and was reviewing some final ideas in my head.

I have a tendency at times to be the person striking up the conversation. I never take offense when someone doesn't reciprocate. I understand. In my case it's not so much an intentional thing, I do it spontaneously. Sometimes conversation spark, other times they don't.

I would like to say yes, but odds are I would not. I would smile and nod, then turn away slightly to express disinterest in making small talk. I do make eye contact with people as I am walking, and I enjoy exchanging smiles with passersby, but I am not a fan of idle small talk, especially about the types of inane things that elevator small talk with a stranger will generally consist of. I always feel a little uncomfortable with it.

It would depend on the circumstances and the person who tried to strike up a conversation. There is a psychology to elevator interaction that has nothing to do with what you want to do. It's an inbred response to the situation.

"If the elevator is crowded, everybody stands still and stares at the ceiling, the floor or the button panel as if they've never seen it before. If two strangers ride together in the elevator, they stand as far as possible from each other, don't face each other directly, don't make eye contact and don't make any sudden movements or noises.

Much of people's behavior in elevators is not the result of rational thinking. It¹s an automatic, instinctive response to the situation. The threat of aggression is not real, yet our mind responds as if it is, and produces behavior meant to protect ourselves.

Elevators are relatively recent inventions, but the social challenges they pose are nothing new. Close proximity to other people in restricted spaces is a situation that has occurred millions of times in the history of humankind. "

http://www.crystalinks.com/elevatorbehavior.html

"Passengers seem to know instinctively how to arrange themselves in an elevator. Two strangers will gravitate to the back corners, a third will stand by the door, at an isosceles remove, until a fourth comes in, at which point passengers three and four will spread toward the front corners, making room, in the center, for a fifth, and so on, like the dots on a die. With each additional passenger, the bodies shift, slotting into the open spaces. The goal, of course, is to maintain (but not too conspicuously) maximum distance and to counteract unwanted intimacies—a code familiar (to half the population) from the urinal bank and (to them and all the rest) from the subway. One should face front. Look up, down, or, if you must, straight ahead."

http://scatter.wordpress.com/2008/04/18/the-social-psychology-of-elevators/

In my experience, unless they know each other, most people will rebuff any conversation with strangers in elevators. It's just human nature.