I tend to see E/I partially about where one's comfort level lies. Most people can think of social situations in which they'd feel uncomfortable and therefore might not say much. I think introverts tend to hit that point with fewer people (especially people we don't know well).
I think I come across as gregarious when interacting with a small group of people I know well. However, put me in a room with people I don't know, and I become a withdrawn observer. I think an extreme extravert might never hit that point unless placed in a completely foreign social context.
One of the more convincing bits of evidence for their being dichotomies comes from a study (MBTI Manual, 1998, p188) in which they plotted faculty rating of gregariousness vs E/I scores. There is a large gap right at the E to I cross-over where gregarious drops suddenly (although more extreme introverts are rated as less gregarious still).
And, like many INFPs, I don't consider myself particularly "emo," nor do I spew emotions publicly. I do feel strongly about some things, but I only share those feelings with those I trust. Often I don't share my "growing edge" of exploration/interest much at all, since I feel particularly vulnerable in that area. What I share is often what I've already settled internally.