The first round interview is more about getting to know you---your personality. It isn't so much about anything related to work---just questions about your background and interests. One thing you really need to do is show flexibility and that you enjoy being adaptable to new work situations. This interview is done outside the office, traditionally at a college campus. Don't worry about any technical questions. PWC's philosophy is that they can give the technical training--the interview is to determine whether you'd be a good "fit." It is always good to ask about the interviewer's history with the firm--they often have very interesting stories to tell. Do research beforehand about the firm--there is plenty of information on the website, but avoid asking about controversial issues such as impending litigation. It is entirely proper to ask about the economic downturn and how it is affecting the firm. If you are a strong candidate you will hear right away about the second round interview.
For the second round, you go to the firm office. An associate will show you around and take you to your two interviews. One interview is usually with a partner, and the other with a director. These will be more the traditional "behavioral" questions, usually asking about teamwork and leadership skills. Again, the main trait you want to show is adaptability. Have specific, recent examples of how you have demonstrated this. If you are invited to the second interview, you have a very good chance of being hired.
You will be taken to lunch by the associate and will be with another interviewee, another associate, and two senior associates. This is a good opportunity to ask about the details of life at the firm [pay, parking, perks, etc.,] Don't ask about these sorts of things at your formal interviews. Remember that everyone you come into contact with is asked about their impression of you.
The second round should also be viewed as a chance for you to gather information about the firm, and to determine whether it's the right place for you. Remember that at this stage you are interviewing the firm members at the same time.