To be necessary, it must never leave the characters it concerns in the same sentiments they had before, but still produce either love or hatred, etc. Sometimes, the change consists in the discovery, sometimes it follows at a distance, and sometimes results immediately from it; the last was used, for example, in Oedipus Rex.
To be effective, it must alter the emotional state of the characters involved so that they end up feeling either love or hatred. The change can occur through revelation, at a distance, or immediately as a result; the last method was employed, for instance, in Oedipus Rex.