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: little test at college
Number of interviews: 2
Skype interview: No
Time between each interview: 1 day
Length of interviews: 20-30 minutes each
Both interviewers asked me about why I wanted to study Biochemistry. From this they asked me a question about why I thought genetics was important as I had mentioned this. They then proceeded to question me to find out a little bit more on what was in my personal statement. This was useful as it allowed me to relax into the interview experience. After this stage, in both interviews the question stage began.
My first interview focused around a written question with some images, related to a little test that we had done the day before. The questions began with A level basic knowledge, which I initially stumbled over in one, yet was corrected, reassured and then continued. The questions got more difficult and challenging, but they corrected and guided me. At one stage, they compared one chemical to another which I had not yet studied. I mentioned this, and they then changed their comparison to help me understand.
I asked my Biology teacher for a