If the signals say 'scary and disapproving' they're hardly expressionless, questioner.
I'm presuming you're the pretty girl and the guy's someone whose signals you're finding difficult to read. If my presumption's wrong, correct me; but it can be that:
A) The 'poker face' doesn't want to express desire / appreciation, which can be because:
i) he thinks you'll learn too much about him and get the upper hand
ii) he thinks you're stuck up and he doesn't want to be 'another number'
B) He can't decide if you're too beautiful to talk to, or too shy for not coming and asking him instead of Y!A
C) His face is paralysed by fear and would never imagine that you had any inkling of interest in him, so his expression is frozen in time.
In any of the above alphabet, if you give more clue, he'll give more answers. The most wondrous times in my life have begun when beautiful women I thought I had no chance with, actually gave a signal that I might.
Or to put it short: smile, and he'll smile back.