This was a question specifically based on some data management work I had done for another organization. With some technical background, he prodded around how I would actually solve a problem in extracting and manipulating data.
I knew my experiences back and forth. I created stories, using STAR (situate, task, action, result) formatting, and told the partner in an engaging way. PwC now has an online help tool with video examples of students answering questions. It has good and bad examples so I personally saw how my answers stacked up.
Core lines of service you should easily know by the final interview stage. I got a feeling for them by their website. Looking closely through the annual review, I saw a few things that PwC was working on that year and spoke about it. It's always good to relate it back to your own thoughts on the subject and also contextualize it within the greater sphere of business and business technology.