Looking down
Looking at a person can be an act of power and domination. Looking down involves not looking at the other person, which hence may be a sign of submission ('I am not a threat, really; please do not hurt me. You are so glorious I would be dazzled if I looked at you.')
Looking down can thus be a signal of submission. It can also indicate that the person is feeling guilty.
A notable way that a lower person looks down at a higher person is by tilting their head back. Even taller people may do this.
Looking down and to the left can indicate that they are talking to themselves (look for slight movement of the lips). Looking down and to the right can indicate that they are attending to internal emotions.
In many cultures where eye contact is a rude or dominant signal, people will look down when talking with others in order to show respect.