Oh, yes, the kids on my elementary school bus used to refer to me as "Stone Face," because I was always staring out the window.
That said, I do have some opposing forces inside. If you catch me in thought, I'm definitely stony, and I do frown a lot. Sometimes it's because I'm upset, but a lot of times it's just because I'm puzzling something out. However, I can become extremely animated when I'm describing my thoughts to other people - well, if they're the sort that are worthy of hearing them.
And over the years, I've learned how to put on an extroverted shell (mainly out of fear of other people not liking me, and just needing some of it to function in a work environment), which I've been doing for long enough that I don't even have to think about it anymore. Really, it's to the point that people will swear up and down that I'm an extrovert and are outright shocked when I tell them that I'm not. The amount of mental and emotional energy it takes is astounding, though, so I avoid social interactions whenever possible.
Then you have my ancestry. I'm half Italian, although it's clearly my dominant side (physically, emotionally, and mentally). And I have to say that as much as I hate stereotyping generally, there really IS a flair for the dramatic that permeates my entire family on that side, as well as most other Italians I've known. We're expressive, passionate, talkative, often loud, and use lots of hand gestures and exaggerated facial expressions. So that part seems to balance me out a bit.
Oh, and as for my experiences, having lived for 40 years with feelings of isolation and being unappreciated, as well as dealing with mental illness (depression and bipolar), I'm very prone to showing signs of intense emotion when I'm with someone "safe." Which pretty much means my husband and occasionally my sisters. It's a side that nobody else sees, though.
So I guess I'm a bit mixed up!