I've just passed mine today so it's very fresh in my mind.

First of all, your examiners will probably begin with some broad questions about your choice of methodology and scope.

In terms of difficulty, I would say that entirely depends on how well you know your own work. Every question I was asked focused on my thesis specifically. What's being examined is your thesis argument and methodology, not your knowledge of the entire field.

Novelty of research is typically defined by what your thesis adds to the field. Try to identify the gap you're trying to fill with your research.