I just had my psych eval last week and thought it went great. I really don't know what you would need to avoid saying. They are basically just making sure that you fully comprehend the surgery and the lifestyle changes you need to make. And that any psych issues (depression, anxiety, OCD, etc) are under control. She pretty much just went over my relationship with food, my family history, and my previous depression issues. She did, however, try to talk me out of the band and into bypass because of my "relationship" with grazing, since I am always on the go with two toddlers. But, I just don't think I can deal with how invasive that procedure is.
Good luck with your eval. I am meeting with my surgeon Tuesday to go over all of my test results, etc, so I am getting anxious!