In order to do it well, they have to become aware of things about their family and its members that they could not possibly have known before. Last, every family is different, so I can't just tell them right off how to proceed. Therapy is a complex process by which the right interventions are devised prior to any actual attempts at implementing them.
For this post, I invoke the principle of opposite behaviors: talking too much about something— especially if one always goes about it in the same way—is as futile as not talking about it at all. In either event, nothing gets resolved.
Telling the other person what they are feeling rather than asking them what they are feeling is another well known example.