2x interviews (1 hour each; 2 interviewers in each)
Discussed experiments; looked at graphs; personal statement
Did not do much preparation; talked to some ex-students
Interviewers don't want you to fail
Remember this advice isn't official. There is no guarantee it will reflect your experience because university applications can change between years. Check the official Cambridge and Oxford websites for more accurate information on this year's application format and the required tests.
Also, someone else's experience may not reflect your own. Most interviews are more like conversations than tests and like, any conversation, they are quite interactive.
I had two interviews, roughly an hour apart and about half an hour long. They were both in the morning and I had arrived and stayed at the college the night before. In both interviews I had two interviewers.
We went through a few experiments, looked at graphs, and talked about a few items on my personal statement.
I did not prepare in any great way.
I wish someone had really emphasised to me that my interviewers do not want me to fail. They are not trying to catch you out but just want to see how you think. I feel knowing this would have removed some of the pressure.