"Even the simple act which we describe as 'seeing someone we know' is to some extent an intellectual process. We pack the physical outline of the person we see with all the notions we have already formulated about him.. In the end they come to fill out so completely the curve of his cheeks, the line of his nose, they blend so harmoniously with the sound of his voice as if he were no more than a transparent envelope, that each time we see the face or hear the voice it is these notions that we recognise and to which we listen." Proust
In effect, the person becomes a convenient and naturalised sign of the notions we have about them; those notions are suddenly congealed and localised in their face, like a meaning tucks itself into a word, so that the word appears to vanish into it.
Sometimes, of course, in 'seeing someone we know' a detail will appear, or confront us, so at odds with this picture, that we will disavow it as insignificant or 'out of character'. In fact, this little 'punctum' represents the ambushing of the imaginary by the real.