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: History Aptitude Test (HAT)
Number of interviews: 2
Time between interviews: One day
Length of interviews: about 30 minutes
Online interview: No
The first interview was to talk through a source that I had been given about 20 minutes to look at before my interview. We then purely talked about what was in the source and the kinds of inferences that could be made, the strengths and weakness and broader themes that came from it. There were no questions about why I wanted to study history etc, this was purely about my source analysis skills. There were two interviewers. One who just made notes and one who asked the questions and had the discussion with me.
My second interview used the written work I had submitted as a springboard to talk about the things I was studying in my history A-Level. We essentially discussed the kinds of questions and debates that would be explored in an A-Level essay, just in more depth and with back and forth discussion. Again there were two interviewers. Each led about half of the interview as the two parts of my A Level studies roughly equated to their areas of study.
Both interviews were actually very enjoyable and relaxed, and just interesting chats rather than intense scrutiny on me. The focus was very much on my understanding of things of what I had studied and had prior knowledge of rather than trying to catch me out or anything like that.
The best preparation was having conversations about my subject with my
Practice papers, discussion with teachers.
I think that having the mindset that the interview is not designed to break you through scrutiny, but give you the opportunity to sort of show off your knowledge and passion for your subject helped me be less nervous etc. I found that plenty of practice just talking about my subject, my studies and debates around that with teachers was the best preparation as it gave me confidence that I could happily discuss most things.
Another bit of advice would be to see it as a discussion where you don't have to give the perfect answer but can offer what you know or think and then discuss and explore it further with the interviewer offering their opinions and ideas as well to help stimulate more from you.