Milgram experiment
Much was done to trick the subjects into thinking it was real. When they arrived, they were told that the actor was another volunteer and that the "teacher" and "learner" roles would be decided at random by drawing slips of paper. In fact, both slips of paper said "teacher", so the actor pretended to have picked "learner". The electric shock machine made buzzing noises and could give an actual mild shock of 45 volts. The teacher would try this out before the experiment began.