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.
Test taken: Geography Admissions Assessment (GAA)
Number of interviews: 2
Skype interview: no
Time between interviews: several hours
Length of first interview: 30 minutes; Length of second interview: 30 minutes
I was given an article to read beforehand and had some time in the library to look at it, then the first half of my first interview was talking about that; summarising it and talking about the main points. Then we talked about my A-Level subjects and how they linked to my degree subject choice.
We then talked about what I had written in my personal statement, although we only focussed on one of the books I had mentioned. They also asked about any other reading I had done, so
My second interview was all based on some sources they gave me in the interview, which I was asked questions about my interpretation of.
In both interviews, there was a very friendly atmosphere. I actually enjoyed both of them; the interviewers were really nice and it was fun to talk about a subject that I was passionate about! I think letting your passion come through is important, the interviewers seemed to respond really well when they can tell you have a genuine interest.
Practice papers from the Cambridge website; mainly the multiple choice questions (under timed conditions)
I came from a disadvantaged background, and the key thing that helped me was getting onto summer schools - especially ones aimed at Oxbridge.
There is the Sutton Trust scheme at Cambridge and UNIQ at Oxford; additionally I have heard there is one at Eton College (which you can apply for a bursary for) is very good. These give you interview practice which can be more of a challenge for state schools to provide.
If you can’t go on a summer school, watch YouTube videos of how the interviews are held. Practice speaking passionately about your subject, maybe give your family and friends a few questions to ask you and talk through each one; try and keep talking rather than thinking in silence!
Make sure you know your personal statement well too, and try and do a bit of wider reading past what you have put down. You don’t have to read full books - you can read academic articles, online newspaper articles, go to any free public lectures at your local university, and watch TED Talks to give you a broad range of things to talk about.