Also, the whole gag probably originated from the way portraits were often painted back in the 1800s and very early 1900s. Many artists did this as a gimmick that was fashionable then. The canvas was actually recessed in the eye areas so there was an optical illusion created and the eyes really did seem to follow one around the room. So, in the earlier days of cinema, many people in the industry grew up with such paintings in their homes and were familiar with them.