As a current student as well as recent interviewee I'm fairly expert on this The interview is divided into three parts: first they ask you generally about your interests and past work, then they discuss your proposal with you -- methodology, approach, impact etc -- and finally they try to identify with you your training needs -- hands-on archaeology, epigraphy, German, numismatics, you name it.
Let me stress that the final part is IN NO WAY intended to spot your weaknesses and use them against you: admitting that you can't even read German, for example, will not impair your application, since the Faculty is well equipped to cater for that need once you're in. The point here is assessing your own ability to identify the areas you need to work on, especially with respect to your future research.