2x 20 minute interviews
First interview - Politics and International Relations focus; Second interview - Sociology and Biological Anthropology focus
Student Room breakdowns, blogs and vloggers, email colleges
Don't overthink it when it's over
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.
My interview was in mid-December. I was taken to a 'sign-in' room where I was given a map with my interview rooms and Common Room/waiting rooms marked. I had two twenty minute interviews in college, both with two fellows. Despite being nervous, the interviews did turn out to be relatively informal - and no where near as intimidating as I imagined them to be.
My interviews were super interesting and varied. After being invited to interview we had to send in a list of four subjects out of the HSPS tripos that we thought we wanted to study for the interviews to use as a guide for questions. My first interview focused a lot more on Politics/International Relations and Social Anthropology. Questions centred around linking abstract questions and issues to real life societies (past and present). The second interview was much more intense. It focused on Sociology and Biological Anthropology (the latter of which I knew nothing about) and the questions were trickier and complicated. I came out of that interview feeling a bit meh. Overall, the interviews (in hindsight) were super interesting but difficult. The questions are really designed to make you think abstractly and about things you might never have considered before, testing your limits, not necessarily your knowledge or the 'right' answer.
The Student Room was super helpful when I applied to Cambridge, they have a very comprehensive breakdown of colleges and sometimes subjects! I did a Sutton Trust Summer School (for English at St. Johns) which allowed me to see how a course at Cambridge would actually be like - it also highlighted that English Literature wasn't the course I wanted to do at uni, so I looked towards HSPS.
I didn't do this at the time of my application but there are some really good blogs and vloggers that give a view into Cambridge, so they might be worth a look. Don't be afraid to email the college with questions - they are there to aid you with the application process!
Once the interview is over, it's over. Don't stress about the result - you can't change anything once you walk out of the college. After my interview I picked at the most tiny of 'mistakes' which obviously stressed me out. You can never really know how the interview goes.