From what my classmates have said, it's unusually laid-back and more like a conversation than an interview. Mine definitely was, but the Bennington interview was the only one I sheduled so I can't compare.
I would say, be prepared to ask a lot of good questions. Remember you're interviewing them as much as they are you. It's too intense and insane a thing (no matter where you go) to be at all happy if you're not sure you have a good fit. In general for postbacs, the worst thing you could do was try to tell them what you think they want to hear, and the worst thing that could happen is you'd get in to someplace it turns out you hate.
That effect would probably be magnified in as small a program as Bennington. So just be honest, tell your story, and if it turns out the way we do stuff is not what you'd do well with, then chalk it up as a win. If it seems you would like the environment and the style, then you can show that better with an open, honest talk.