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.
Number of interviews: 2
Skype interview: No
Time between each interview: 1 day
Length of interviews: about 45 minutes each
In my history interview, the discussion was centred on the essay I had sent in as an example of my written work. They had told me to expect this, and they also ended up asking me to link some of the themes we discussed to other periods of history I was studying/had studied.
For my economics interview it was problem-based, so I was not expected to have any prior knowledge, was presented with the beginnings of a problem, and was supported to reach some answers and talk about my thinking throughout. I felt like both interviews were fair.
I looked at past papers for the