A. Most models have certain things they are not comfortable participating in, or certain kind of pictures they are not interesting in publicising. I never try to make models do pictures they are not interested in doing, and I always respect personal limits. I do prefer to know these in advance as I often have a very good idea on what I want to achieve in a certain session. Knowing personal borders in advance often makes the difference between a good session where we both are comfortable with what we do and one where you as a model feel I try to make you do something you don't want to do.
A model that constantly says NO when asked to pose in a certain way is not a dream scenario for a photographer, and a photographer that constantly try and push limits can be annoying for a model, both are mood killers. Limits are fine, but let's get them out in the open in advance so we can enjoy the session and make pictures we both like.