In some of my interviews I had to test a pen, login page, etc., In such cases always ask questions like what is the purpose of the product, what does it do, how much time do you have, think of positive and negative cases, run through the scripted and non scripted test cases (if possible).
One of my interview went really interesting, that I was given an architecture of entire system in which questions were asked to know about my assumptions and my exposure towards such systems.