Winston considers the thought of the dark haired girl being an amateur spy which brings this issue to the audience. He notes that an amateur might be particularly dangerous because you have no idea how long they have been looking at you, or if they are really a spy. Having to control your facial features to ensure that they might not be misconstrued would be a further difficulty with a newer spy. They might read something in you that is not really there. Someone might be a person you have known for a while, or they might be sitting on the next table over and watching you every day. Winston's greatest fear within eyesight of an amateur spy would have been his gestures:

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.