The subject of conversational hypnosis is very controversial. Most would argue that one cannot be hypnotized without knowing about it or consenting to it. Most types of hypnosis actually do require the agreement of the person being hypnotized. The idea of hypnotizing a subject without their knowledge of it is absurd. However, with the proper training you can have a powerful influence over others using conversational hypnosis.
Hypnosis is simply a state of awareness. Your conscious mind is not filtering the ideas that enter your subconscious mind. Typically, you can get to this state of awareness through deep relaxation and guided visualization techniques. This is the most common way that people use to enter a hypnotic trance. However, these methods, unlike conversational hypnosis, require your participation and awareness.