4 For the proper use of these media it is most necessary that all who employ them be acquainted with the norms of morality and conscientiously put them into practice in this area. They must look, then, to the nature of what is communicated, given the special character of each of these media. At the same time they must take into consideration the entire situation or circumstances, namely, the persons, place, time and other conditions under which communication takes place and which can affect or totally change its propriety. Among these circumstances to be considered is the precise manner in which a given medium achieves its effect. For its influence can be so great that men, especially if they are unprepared, can scarcely become aware of it, govern its impact, or, if necessary, reject it.
Source: Inter Mirifica (Vatican.va)