Is it considered backbiting to say what so-and-so said, or to anticipate what he will do, or to express my feelings towards him, or to interpret his words and actions, or to say bad things about him that I could confront him with?

Backbiting is to mention your brother with what he dislikes. If what you say about him is true, then you have backbitten him; otherwise, you have slandered him. Spreading كلام with the intent of causing mischief is forbidden calumny (namīma). Disclosing words spoken in a gathering is a betrayal of trust and is impermissible. As for entertaining ill thoughts about a Muslim whose uprightness is apparent, it is not backbiting, but it is nevertheless impermissible. Reporting what someone intends by their words and actions is impermissible because it is speaking about the unseen, unless you are overwhelmingly certain of it and there is a clear benefit in disclosing it. Reporting your feelings towards someone is not backbiting, and it is preferable that love and hatred be for the sake of Allah. The fact that a person is able to mention another with what he dislikes to his face does not justify mentioning him with it in his absence, because harm is forbidden in all circumstances.