It's not really possible to "fail" the interview. The interviewers at that point know that your work would fit the program and are sussing out whether you are good fit for the community. As long as you show up engaged, and willing to answer questions, you'll be fine. The week before, have a few studio visits with people who have never seen your work before - this helps work out jitters.
As for writing your intent, I find it's useful to talk about how you want your work to grow over the next 2-3 years, and how grad school is an important part of that progress. What do you think you'd get out of participating in this program in particular? I strongly recommend mentioning 1-2 faculty members you particularly want to work with.