“At our school they’d reject after interview if kids were rude, or silent/couldn’t be drawn into discussion, or came across as likely to be disruptive to a class, or came across as being coached within an inch of their life and giving scripted answers”
This is pretty much what we were told by the head of admissions at the school DS went on to join. The impression I got was very much that passing the exam was the biggest hurdle and they were looking to make as many offers as they could, and were ruling out candidates at interviews, rather than candidates having to rule themselves in IYSWIM.
That said, for some of the super selective London indies the interview may carry more weight.
Everyone I talked to about it - admissions people, teachers who did the interviews, our tutor, agreed that it was possible to over prepare and this put kids at a disadvantage. They want to see kids as themselves not a scripted version.
My favourite example is a friend’s son who was asked in interview about his favourite foods. He said his favourite thing was a dessert served on the open day of another school he’d applied to (he named the other school)! He got offers from both schools.