Observing the thoughts

What exactly do you mean by “observing the thinker”? If someone goes to a doctor and says, “I hear voices in my head,” they will most likely be sent to a psychiatrist.  But the truth is that everyone hears some voice or other in their head, literally all the time: it is this unintentional, unprepared, and automatic thought that you don’t even know you have the power to stop, this endless utterance or conversation that you actually have the power to stop.
You may have seen a “madman” walking down the street, talking or muttering to himself. But, there is no special difference between what the madman is doing and you and other “normal” people, except that you do not speak like him. This voice speaking in the head comments, speculates, makes judgments, compares, complains, likes and dislikes, etc.  It is not necessary that the voice is speaking to you wherever you are or whatever situation you are in.