In the movie "The Conversation", the real plan of the couple is as follows:
Martin Stett had put the idea into the Director's head that his wife was cheating [...] and hired Caul to record them. The conversation in the plaza therefore, was a ruse. The young couple is merely pretending to plan an illicit rendezvous at a hotel when in fact, they are setting up the Director to arrive and confront them, [Wikipedia.]
This doesn't make sense. Why didn't they think of an easier plan, and why did they involve someone else (Caul) in the plan (who could endanger it)?