From Glossary in Bella Merlin's book on KONSTANTIN STANISLAVSKY in routledge performance pracitionners

subtext
This term refers to the motivations which underlie the surface of
the spoken text and external action. A character’s subtext can be
expressed through specific intonation, looks, gestures, pauses, or
stillness. As Stanislavsky puts it: ‘Keep in mind that a person says
only ten per cent of what lies in his head, ninety per cent remains
unspoken. On the stage [we] forget this, [we] are concerned only
with what is said aloud, and thereby destroy the living truth.
Playing any scene, you should first create all the thoughts which
precede this or that cue. You don’t have to express them but you
have to live with them’ (cited in Toporkov 1998: 181).