I have 2 of them; DD12 and DD7. But, interestingly enough they are pretty good at masking some of their emotions in public but not at all at home or with close family friends, and their emotional reactions sometimes still take DH and I by surprise.

One of the things we figured out is that if we try to minimize their emotional response with phrases like "calm down, you're over reacting, just relax, you're just going to have to learn to not be so sensitive" or other such lines that someone pre-programs you to say to kids it only makes things 100 times worse.
We have changed our words to things like "are you sure that your reaction is the level that this situation needs to have" or on a scale of 1 - 10 where is this incident and so what should your reaction level be" and other things that I never thought I would say because they don't really sound like me.
We have been teaching the girls how to internally think about the situation they are in and figure out how much of a reaction they should/need to have. When we got them thinking about whether or not they really needed to have as much of an emotional reaction to things they began to learn how to better deal with their intense emotions. Does it work all the time - NO way, but it helps more often than not. It has also allowed them to understand that their intensities are not bad things, just things they need to learn to control and not things that they should always allow themselves to be controlled by.