In LIS, some non-manuals can also contribute in distinguishing specificity from non-specificity. In particular, raised eyebrows (re), wide-open eyes (we), and relaxed mouth-corners together with a backward head tilt (ht-b) may trigger a specific interpretation, namely the signer is talking about a discourse referent that she bears in mind.
ht-b
we
re
persona ixa friendb bseizea
‘A person (I know who) kidnapped a friend of mine.’
As for non-specificity, the facial expressions accompanying non-specific reading corresponds to those used for indicating indefiniteness, basically they are a backward tilted head (ht-b) and mouth-corners down (md).
md
ht-b
someonea ixb friendb ixb carb possb bseizea
‘Someone (I don’t know who) stole the car of a friend of mine.’