The thesis defense is a dialogue about the common subject at hand — your scholarly and critical work. Your thesis is the shared occasion for a congenial but rigorous discussion, not a mean-spirited inquisition designed to trip you up.
Remember: If you have done your work well, you will likely be better informed about your topic than the members of your thesis committee. You should be able to talk about your thesis articulately and to explain why you did what you did, why you arrived at certain conclusions and rejected others, what sources you considered and, perhaps, why you didn't look at certain material or go down alternative avenues of inquiry — why, for example, you found one collection of documents more useful and reliable than another.
The defense typically takes one hour.