I know that sometimes, when thinking, I have a sad/tired look on my face because of the ever-present shadows under my eyes (they're there even when I've had lots of sleep!) and the natural downward turn of the corners of my mouth. But at the same time, I could easily be grinning about something I just thought of.
I know that I have a very expressive face that usually betrays how I really feel about something...especially when I'm not aware of how I feel at the time. It's very frightening for me for someone to tell me the expression on my face and it be exactly a reflection of what I was thinking at that moment. It's frightening, because the only time I'm aware of the expression on my face is when I'm purposefully trying to show an expression for some kind of effect. So, most of the time, I don't know what my face looks like at any given moment. Even if I'm smiling, I don't always know what else might be communicated. It's because of this that I'm a terrible liar.
When around other people, I smile and laugh a lot because there is so much information I'm receiving that a lot of it ends up being very amusing...aside from someone being outright funny.