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: Biomedical Admissions Test (BMAT)
Number of interviews: 4
Time between interviews: Between 15 minutes and 2 hours
Length of interviews: About 25 minutes
Online interview: No
In most of my interviews, I was shown some diagram or graph that I was asked to explain - they usually had some labels/hints as to what they were, so I talked about each label and hint, then tried to put them into context. I was asked once about my personal statement, but I hadn't prepared well enough to back myself up - I had explained in my personal statement that I had found something interesting, and they asked what I had done to find out more/ follow it up, and I had little to say. I had 2 or 3 interviewers in each interview, which was intense but some were nice and others not so much. I did not really relax as the time does fly, but the conversations we had were very valuable and I reflect happily on them.
I was given a mock interview at school the day before, which I used to practise explaining my ideas out loud and this was really useful. I highlighted each part of my personal statement that made any kind of claim/said anything new, and then created mind maps of each of these sections so that it was fresh in my mind. The best thing that I did was each time I read a book or article, I would almost enact an interview in my head - I would think critically about what was being said, try to explain it in my own words and try to put the information into the context of further experiments. This was so hard at first but got much easier, and made me more confident going into my interviews. I also tried to look ahead a bit at the biology content, which was useful as the interviewers asked me something beyond where I was at the time in A-level biology.
I found an old BMAT advice book in my school library, which I took practice questions from. All the information for preparation is on admissionstesting.org - i downloaded the syllabus, highlighted parts i was less sure on, and did lots of test questions under timed conditions. I did a bit each day, including reviewing the content regularly.
Don't be afraid to ask the interviewer to repeat the question or clarify what they mean. Tutors see many students every week - this is their job, so don't be afraid that you're going to say something silly as they deal with students all the time. When asked a question that you have no idea how to answer, just start with what you know - explain the key words and phrases, and be brave enough to make suggestions and experiment aloud. The interviewer will guide you away from bad ideas, but only if you're talking through those ideas out loud.