A nervous tic, an unconscious look of anxiety, a habit of muttering to yourselfanything that carried with it the suggestion of abnormality, of having something to hide. In any case, to wear an improper expression on your face (to look incredulous when a victory was announced, for example) was itself a punishable offence. There was even a word for it in Newspeak: facecrime, it was called.
Orwell saw it coming!
He absolutely did.