In one classic investigation the communicator was labeled as the editor of Pravda or was an American physicist. Greater attitude change was obtained with the American communicator. This manipulation works especially well when forewarning is given with sufficient lead time. The next wave of research, based on cognitive dissonance theory in the 1950's. Attitude change as a means of reducing dissonance.
People sometimes engage in attitude discrepant behavior.
Later they contemplate what they did and experience dissonance. If the behavior is performed under strong external constraints in the situation, then there is little dissonance aroused and therefore, little change in beliefs. If the constraints are weak then there should be larger change in beliefs. The principle is that less is more. Use the weakest influence that you can to still get the person to perform the behavior.