"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. In short anything that carried with it the suggestion of abnormality or of having something to hide.
In any case, to wear an improper expression on your face was itself a punishable offence.
There was even a word for it in Newspeak: facecrime..."
Orwell - 1984