Why is the candidate hoping to get out of the experience?
I think you ought to be really honest and just say, look, this is just interview practice - it might be totally different from what you experience at Cambridge, so please don't be thrown. We're just practising having a conversation.
There is loads of good advice on interviews and what to expect on The Student Room. Sometimes they have admissions tutors on there giving advice.
That said, I've done admissions at Cambridge and I think yes, it's common to discuss an extract or a text. Questions tend to be open-ended; IME this is sometimes something that throws candidates who have been taught to believe there is a single right answer and you must not deviate from it.
I don't think I can see myself asking 'why do you think he made these changes'? I might ask 'what effect do these changes have'?
Basically, you want to be asking open questions that will require the candidate to pay close attention to the text, and justify what they are saying.
Ideally, the text(s) you discuss ought to be reasonably obscure. It's fine if the candidate happens to know them, but they should not be on the syllabus (!) of any exam board.