In order to answer this question, we have to first ask what suggestion is, what an induction is, and how much of hypnosis is an art as opposed to pure science.
Let's take the question of suggestion first. If suggestion is the is the transmission of information in order to change one's views of reality (the self, the world, and the future), and hypnosis is the use of suggestion to accomplish these aims, then all communication may be viewed as hypnosis, even though it varies in the degree of its effectiveness.
An induction usually helps, because when you accept the expressed or implied suggestion that your mental processes are beginning to function differently (i.e., that you are hypnotized), it's easier to depart from the established "rules of evidence" that are inferred from your past experience and laid down by the society in which you live which enable you to decide just what is real and what isn't.
I like to think of hypnosis as a catalyst rather than a cause. When you have accepted the suggestion that you are hypnotized (as you understand it}, other events can occur more easily. In different settings (such as in your office vs. in a classroom), with different people (Client X vs. Client Y), and the use of different suggestions (suggestion A vs. suggestion B). you will, of course, get different results, some of which can be quite dramatic. That's where the "art" comes in (and of course, it's also an art to suggest that one has been hypnotized!).
Although covert hypnosis (sometimes referred to as "ambush hypnosis") can work occasionally with an unsuspecting person who is caught by surprise, much more often than not, people will catch on to what you are trying to do. They will either laugh at you, or become angry for insulting their intelligence, and/or suspect that you have an ulterior motive and report you as a suspicious person -- that is, if they don't decide to take the matter into their own hands and give you a "knuckle sandwich!'. And if anyone should still believe in the validity of "ambush hypnosis" (via NLP or any other means), tell that person to stop trying to cut in and go to the end of the line!