Final round of reviewing for a Program Committee I've been recently involved in a Program Committee, and I was wondering what exactly was expected of the PC members from the PC Chair. Once all the reviews have been collected, are the reviewers expected to engage in a discussion about the paper? Or do they need to wait for specific questions from the PC Chair? Are the reviewers expected to comment on the other reviews? 

In case it matters, we are using the EasyChair system, so everything is done online, and I don't know the identity of the other reviewers.
In my experience, usually the PC chair sets expectations by providing explicit instructions and guidance on the process.

In my field, the deadline for submitting all reviews is usually a week or two before the PC meeting.  That last week or two is reserved for a "online discussion phase".  During the discussion phase, reviewers are expected to read the other reviews on the papers they were assigned and discuss what to do on each paper, to try to reach consensus on the final discussion.  This might involve some discussion on points where reviewer opinion differed or on technical points of disagreement, for instance.  Each reviewer is expected to take the lead on beginning the discussion, without waiting for the PC chair to prompt you or ask you a specific question.  The goal is to try to reach a clear decision before the PC meeting in as many cases as possible, or at least reach agreement on what the strengths and weaknesses of the paper are so that a final decision can be made on an informed basis.

If there is no in-person PC meeting, the online discussion phase becomes especially important.

But that's just my personal experience and my field.  Practices may vary in other fields, for all I know.  If your PC chair did not instruct you on what he/she expected, it is completely reasonable to contact him/her and ask what he/she should like you to do.