Practice being comfortable with an adequate amount of wait time. Use review questions from the last session or even a nonrelated current event to get things going - the more controversial the better. You might even want to look at The Book of Questions. It poses dilemmas of all kinds and asks readers to take a stand. Go around the room and let every student comment.
Students clam up as soon as they think the question posed is a test of their intelligence.
So let's consider all the ways we might go about figuring out in which period to place this skull. There are lots of ways the story could be interpreted. Let's see if we agree or disagree with the literary critics. Offer different options for participation. Letting students turn in a written response to discussion topics on occasion helps shyer students.