Your subconscious often makes use of faces that you have seen throughout your life. The hundreds of thousands of faces you have seen throughout your life are stored in your subconscious. Your dreams make use of these faces because we dream usually of what we know. Faceless characters in dreams are usually parts of ourselves that we do not know very well.
The important thing to know is that each character in our dreams represents a part of our own psyche. The face we choose to put on a character in a dream represents a quality that is somehow like a part of our psyche. This is at the first and usually most valuable level of dream interpretation.