Generally flat.
I rarely smile, except at work where it is a requirement, and when I do it is obvious it is "unnatural". I was having a conversation with a customer today, and was trying (probably too hard) to seem friendly. My coworker started cracking up and said how nervous I looked while smiling. I wasn't nervous, just incredibly uncomfortable. My natural smile is a barely noticable smirk (even when I find something hilarious, the smirk is the most I do).
The rest of the time my face is generally flat except for the occasional quizzical look which is my first response to most things.
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Those who cannot tell what they desire or expect, still sigh and struggle with indefinite thoughts and vast wishes. - Emerson
Selfishness is not living as one wishes to live, it is asking others to live as one wishes to live. - Oscar Wilde