Top half of face: slightly furrowed brow with eyebrows canted slightly up as they approach center of face. Worried, scared, some flavor of "nervous"
Bottom half: grin, but that can be found in conjunction with the above
Best guess: nervous anticipation, or the onset of a psychotic break
In regards to me: I barely even grin most of the time, never mind SMILE, for any reason. I have to be pretty happy, or someone has to tell a damn funny joke. Facial expression when logged into KF is generally very flat/deadpan, occasionally something really funny can break that but it doesn't happen too often.
the look on an employee's face after he just "took initiative" and "thought outside the box" and thought you would like what he did but the more he talks the more nervous he gets over the fact that you're not smiling