The first thing I would do is to ask the committee chair what are my rights and responsibilities as an advisory member. In short, what is it expected from me. I guess I would be expected to ask questions but I will not participate on the final decision of the defence.
Participation in a PhD defence committee seems to me as a good opportunity to learn critical thinking about research. The thinking every researcher should apply on his/her own work. Therefore, as a preparation I would recommend:
(as you mentioned) Carefully read the dissertation and
- if you find something which is not clear to you even after several readings, ask about it
- look at some of the references you find interesting and try to ask yourself: "Is this reference appropriate here? Why?"
- in case some formulas or computations are present, you can try to reach the same result as the PhD candidate
- if you are interested in methodology you can ask the
candidate why he/she used that particular method
to do more, you can try to find another works on the same topic, read them and compare them with the dissertation as it is always good for discussion to have more information about the topic
In case of public PhD defence, the audience can be very broad from family members who do not know what is the research about to an expert from another department who came to rise his/her ego by asking non-answerable questions. Here, the question depends on the person who is asking. No background knowledge is expected here and one should not feel restricted by the circumstances.
My personal opinion is that one can ask really about anything as far as his/her intentions are good. If the question is irrelevant the PhD candidate or one of the committee should kindly explain why.
