Which role should other graduate students serve in a PhD defense? In some countries and at certain universities, graduate students are advisory members of the committee during the PhD defense. In other countries like the Netherlands, a PhD defense takes place in public and everyone might ask questions at the end. Therefore, I would like to know:


How should another graduate student serve his/her role as an advisory member of the PhD committee?
What kind of questions should one ask in a public PhD defense?
How should he/she prepare for the defense (other than obviously reading the dissertation)?
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.