The physical closeness of the authority figure and the person they are giving the order to.
In Milgram's study, the teacher could hear the learner but not see him. In the proximity variation, the teacher and the learner were in the same room. - The obedience rate dropped from 65% to 40%
In the remote instruction variation, the experimenter left the room and gave instructions over the phone. Obedience reduced to 20% and ppts frequently pretended to give shocks.