"It was terribly dangerous to let your thoughts wander when you were in any public place or within range of a telescreen. The smallest thing could give you away. A nervous tic, an unconscious look of anxiety, a habit of muttering to yourself--anything that carried with it the suggestion of abnormality, of having something to hide. In any case, to wear an improper expression on your face...; was itself a punishable offense. There was even a word for it in Newspeak: facecrime..."
1984, Book 1, Chapter V