In my first stage interview there was a good ammount of commercial questioning in the less formal chat but the vast majority of the interview was focussed on competency based questions.
If you were to grab a couple of copies of The Economist and read The Times / Google News Business section for a week or so you should be fine. I think I was probably over prepared as far as commercial knowledge was concerned.
I was asked questions like,
“Tell me a about a particular company you have had an interest in recently and why has it interested you?”
I was asked about the ACA and the feedback I received was that knowing about individual exams, course structure and ammount of time I would be expected to study etc. was well received.