I would go semi-casual in case they want to conduct the interview on the spot ... I've run into someone who wanted to interview me verbally before they gave me the application to take home so dress semi-casual.
That's a tough one. What kind of job is this for? Sounds like a trick to me. I would avoid being "VERY casual" and go with some kahkis, a nice polo and decent shoes. That way, you look more dressed up than usual but not in a full out suit.