Perhaps the transformative nature of the performances in Cassavetes' work is related to a certain indiscernibility of narrative intention that arises from each character seeming to act out of their own internal logic. By this I mean as characters primarily, rather than as conduits in the service of plot.
I think what you're trying to describe here is the way that the films withhold access to an interiority that often functions to explain a character's behaviour. The performances certainly create a powerful sense of inner turmoil and psychology depth. But the impact is due to the fact that the films never psychologise the characters. Sylvie Pierre puts it best when she describes Faces as giving the impression of "being a thorough investigation which, however, reveals nothing at all". She goes on to note that Faces 'makes us aware of one of the weaknesses of the cinema: its right and proper inability to explain the inner world, since all it can literally grasp are external signs as being not unrelated to inner turmoil'. What Pierre points to is cinema's unsurpassed ability to record and capture the fluctuations of emotion as they pass across the surface of a face. Cassavetes' work is distinguished by a highly sophisticated understanding of how cinema engages with emotion via the image, an image that draws its meaning from the skin, the face and light, rather than some false interiority.
(George Kouvaros interviewed by Needeya Islam on sensesofcinema.com)