Facial expression? What facial expression? Is that one of those things people use, like these "tone of voice" and "vocal inflection" concepts I keep hearing about?
Likely so, unfortunately. I don't focus on facial expression, but can mimic general attitudes to some degree if I think it is necessary. It is hard, though, and I can't keep it up for long. Generally I do an excellent job of being myself, and a poor job of trying to be anything else. If people are uncomfortable with the real me, they don't have to stay around more than absolutely necessary.
I don't show a great deal of emotion. I usually find that if you show people little enough of yourself, then eventually they start to see whatever they want. Thus people often come to believe things about me that simply aren't true. They say that nature hates a vacuum, I guess the human mind does as well.
Because of this I rarely have to bother mirroring people even if I feel there is any reason to do so. I just wait until they tell me who they think I am, then say "Yes, well spotted."
"How dreadful!" cried Lord Henry. "I can stand brute force, but brute reason is quite unbearable. There is something unfair about its use. It is hitting below the intellect." ~ Oscar Wilde - The picture of Dorian Gray