I tend to describe My Friend John as conversational, rather than covert. Personally, I have no desire to practice covert hypnosis on anyone. I always tell my client’s that from the minute they enter my office, the hypnosis has begun, so I can ensure they get good value for money. That means there is no ‘catching me out,’ because I am doing precisely what I am paid to do. I was just doing it in a slightly different way.
This question seems to assume that you will be able to lead your client completely into hypnosis without them at any point registering. Now, although that can take place, at times, it is not the usual response. I would guess that most cases involve the client realising what is happening as they get closer to trance. However, by that point, they are too relaxed or invested in the process to object.