Anyone with even average intelligence and a bit of imagination can by hypnotized to one degree or another. Once hypnotized, if the therapist knows what they are doing, therapy can be conducted in a way that works best for the client.
There is a range of susceptibilty to both hypnosis and hypnotherapy, but in general, "yes", virtually everyone will respond. In some cases, a person may be only capable of only light levels of hypnotic trance, but the good news is that most therapies can be conducted at even Stage 1 on Arons Scale of Hypnotic Depth.
Hypnosis almost always requires a willing participant. If you actively resist or work to prevent trance, then you would be the exception to this question.
No. Most change work can be done at even the preliminary levels of hypnosis.
As long as the subconsious is open to suggestion, work can proceed and that happens in even the lightest trances.
You can see one system for measuring trance depth on my Arons Scale page.
As a rule, the content of a session has been discussed in advance, but it can happen where the hypnotherapist is using imagery you aren't seeing or don't like.
In most cases, if the hypnotherapist keeps saying "green" and you are seeing "blue", you will simply see blue and go on. For most things, the exactness of the image is not the point. If you find it jarring or the hypnotherapist is using an image you can't deal with, you will simply open your eyes and correct what is being said.
I had one client who didn't reveal a phobia to me. With her in a trance, I was using a flock of birds moving in formation in the sky as an image. She opened her eyes and asked if could skip the birds because birds are "too pecky". I changed images and she was back under, more deeply than before.
That's a question that is so important that I dedicated an entire page to it.
Please see the "Choosing a Hypnotherapist" page.