Occasionally I am mistaken for an Extravert. But in reality, I am nowhere near an extravert. My open spontaneity and garrulousness seem to come as a direct result of my surroundings and companions. As others have mentioned, groups sap these qualities right out of me, and I am usually extremely reserved. If I am in a one-on-one situation, or with two other people, my spontaneity can burst through the seams if I believe these companions already understand me well, and I can feel comfortable letting out whatever is inside without judgment. I am very keen in perceiving who is likely to pass judgment upon me, and for what sort of action.
Another reason I can open up is when I get the "feel" that I am dominant in some way. If my companion is more reserved than I am, and I sense that the pilot position is free that I may set the tone and energy for the situation, then I will seize it. This leads to me often having a very zany, almost manic energy. Sometimes I will just start beat-boxing, flapping my arms gracefully and taking flight around a room or outdoors, or perhaps I will take hold of invisible jungle vines and pantomime myself swinging from vine to vine. There is little doubt this is attention-seeking behavior, but I also enjoy entertaining shy people, and knowing that I am creating some perplexity.
In the matter of communication, I may explode into a rapid-fire investigation of a subject, involving lots of colorful speculation, and questions to my companion(s). I like to see if I can engage them with as much fascination as I have for the subject at hand. Sometimes it is a fun challenge to read the other people and do an impromptu interpretation of what sort of communication will spur others into the throes of fanciful engagement. I have a friend who loves theories, but is suspicious of loosely-applied speculation (INTJ), so with inward glee I mentally dart around looking for a theoretical subject to broach that will enrapture his fancy without triggering his suspicions. It takes some occasional mental acrobatics on my part, because he is quick to notice exceptions and point out discrepancies, so I have to maintain strong cognizance. This sort of mentally-involving communication allows me as an INFP to open up and enjoy vivid dialogue and spontaneity.