2x 1 hr interviews, 1 day apart
Interview 1: physics questions, personal statement; Interview 2: motivations
Revise physics, biology and chemistry; do extracurriculars and read the recommended books, but also something unique.
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: 1 hr each
In my first interview I was given a broad question about pressure and gravity meant to test my understanding of physics A level and how I could apply it to a hypothetical question about the Earth. I started with very minimal information, and as I got stuck I was given more prompts to keep working towards my answer and to consider factors I hadn't thought of, with occasional ""okay, but now what if..."" questions thrown in to encourage me to go deeper and think more about the problem in a more complex way. They then went through my personal statement and asked me a very little bit about the books I'd read and a lot about work I'd done on my gap year. This tied into a chemistry and biology related question and trying to work out an equation to answer that question using first principles. I found this interview more challenging mostly because I was quite rusty with the equations I'd learnt at school, having taken a year off, so they didn't come back to me as quickly as would have been nice, but I got a lot of guidance and prompts to encourage me to keep going when I got stuck.
In my second interview I was asked about why I wanted to study my course, how I'd come to that decision and what had I done outside of the classroom to reaffirm this. I was also given a few rock samples to talk about. This second interview was much more of a relaxed chat as I only needed to express what I thought and why as opposed to having to think about equations or more complex problems.
My best advice would be to make sure you have a firm understanding of particularly the more basic physics, biology and chemistry principles - you don't have to have memorised the entire formula sheet from A Level but just like knowing the really fundamental equations from GCSE will make your life a lot easier - or at least how major properties (force, velocity, etc.) are related to each other.
My expectations were that they would try to catch me out or trip me up when asking questions, but the interviewers were actually really lovely and patient and were just trying to help me answer the questions they set, as well as to challenge me a bit. If I could do anything different I would try to relax and stay calm, because panicking in the exam just makes you more quiet and you can't show your true potential.
I really believe that what they're looking for most is someone who is genuinely enthusiastic and passionate about this subject - it's not something you can fake, but to give yourself the best chance of showing this enthusiasm include as much stuff about what you love doing/reading/thinking about (but still relevant to your subject) on your personal statement. That way, the interviewers can ask you about it and see this excitement.