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 (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.
Remember when the Democrats used to be somewhat careful in their public pronouncements to avoid direct comparisons with Orwell’s fiction? Now they practically quote Orwell and Rand villains verbatim.