Hi, I recently had a final interview with PA and was sucessful. I asked for the criteria by which the manager was judging me against (after responding to his questions). I would imagine it's similar for other managers:
1. Presence/ability to communicate your ideas/how you'd come across to their clients
2. What you can offer/relevant experience (I'm not a recent grad)
3. Desire to achieve/ambition
In addition the partner said he was impressed with my questions to him.
Read around the industry (e.g. on this website), are there areas of development or topics that make you want to explore further from someone with experience of the industry? If you can demonstrate your understanding, communicate a concept and show an enquiring insighful mind, this will stand you in good stead.