I treat the interview as my chance to help the candidate pass the interview. This bent of thought may seem subtle, but makes all the difference in meeting the candidate in their terrain, and seeing the world from their point of view. Consequently, the case studies given explicitly state that if the candidate finds something else that intrigues them, I am happy to take that as a case study instead - gives them something to flaunt and for me to learn about.
Some candidates are shy to open up, or just not comfortable conversing with strangers, or misread the power asymmetry in the interview and get anxious - I spend a fair bit of time just conversing human to human.
For the really uncommunicative candidates, I make a slight of what they built (fake slight) and this gets the conversation going like a star. The good candidates exhibit a great amount of "Builder's Pride" and defend what they built. They really good ones admit to the possibility that there were other better ways to have built, or explain to me the constraints under which they made the choices they did.