People that are shy and introverted tell therapists that when they enter a restaurant, people look at them, creating anxiety. Its true, but it applies to everyone, not just those who are shy. When anything enters your visual field, you subconsciously begin scanning it. A person walking into a room is scanned by almost everyone else and that automatic scanning procedure takes about two seconds.
The subconscious mind is looking for two things 1) to see if you have a memory or point of reference for comparison and 2) to protect you for any signs of danger. If the new individual is odd looking, carrying a weapon, or naked, the brain will start a full-scan and react accordingly (long stare, fright, or Dont I know you?). Individuals with physical features that are unusual lead to the common double take where you will first subconsciously scan for safety and reference, then look again consciously to examine and analyze.
These references are designed to help you, as when remembering an old friend, the location of the store in a mall, or when remembering needed facts/details. This occurs subconsciously as reflex and instinct. To override or cancel this natural/normal procedure requires attention, focus, and effort.
Because of the tremendous amount of data streaming in to the mind every second from your senses, your minds ability to perform routine tasks subconsciously is essential. The human eye, for example, scans two billion bits of data per second. If all this data were not already organized somehow, the conscious mind would have to start from scratch to figure out what each pattern of light and dark meant. You simply cant afford to consciously process all the data every time you move your eyes. It would take all day just to get dressed.
Automatic, subconscious processes allow you to respond to familiar situations quickly, efficiently, whereas controlled, conscious processes produce responses slowly, demanding a great deal of attention and mental effort. However, you typically use conscious processes only when you must or are highly motivated to use them.