T-800 is not just a face without expression.
to infiltrate humans better, he has some artificial emotions which it doesn't feel but expresses either randomly or badly programmed.
and i don't mean just "it's an actor and he changes faces", no.
1) the endoskull has little holes all over it.
do you see the places they should be on this copy?
under the nose, under eyebrows, between eyebrows and so on.
those are holes for actual
nerves on an actual skull.
so terminator's face skin is connected to its artificial brain by actual live nerves. that allows the face to act.
i've pointed the ones i managed to notice.
Stan Winston was very keen of physiology, i guess that's why he always made creatures.
thanks to Moguchiy H for explaining me that purpose of actual skulls holes.
2) the artificial head in eye surgery scene twitches it skin very strong while it is cut. the skin itself has nerve pain, though the artificial brain must ignore it because it itself doesn't have receptors for physical pain.
unluckily i cannot put a video of that face meat twitching while nerves are cut. (edit - after, not while, of course). it has to be checked actually watching the scene.
terminator is - or at least was intended to be - a cyborg, not a robot with separate flesh cover. in the first script Kyle Reese mentions that it has small "chicken size" heart, lungs and intestines and has to eat - all that for keeping his live part alive. yes he doesn't need it to function, but his electronic and flesh parts do act as one.
You have been targeted for termination!!!
By the way your custom Bane boots are freaking awesome brotha. Top notch work on my T1 custom boots too. I can't thank you enough for a job well done sir. 
