The witnessing part is passive, but the witness is not the only role that the therapist assumes. They do whatever feels appropriate and best at the moment, whether it's giving support, interpreting what the client said, going deeper in trying to understand the client's material by asking questions..etc.

Actually, witnessing too is not entirely passive. You don't just stare at someone and watch their processes. You empathize with them, let them know that you can see and appreciate what their going through, so it's not just passive listening..In fact, the deeper meaning of listening is never passive. If the other person doesn't know and can't feel if you understand them and accept them where they are, then you are not really listening, you are just pretending that you are listening. That's why I am also not someone who likes to be stared at. If the therapist just stares for a prolonged period of time, I don't experience them as listening. I need some kind of an indication from them that they heard what I said and that they heard it right.