2x interviews (45 mins each)
First interview: chemistry calculations, organics; Second interview: pure maths, curve sketching, experiments
Past papers; Practice interviews; School events
Be enthusiastic; Keep talking; Do your best!
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.
I was interviewed in early December with the majority of applicants. I stayed in the college overnight before the interview as I live a long way from Cambridge.
In the morning I had breakfast in the collage hall. My first interview was a chemistry based one, I was guided to the room by a current student. Whilst waiting for my interview I could just about make out the end of the interview before me (it was not going well). I was (obviously) asked different questions.
I had about an hours wait in-between interviews which was spent in the JCR with other candidates and student reps(who were very reassuring in my panic). My second interview was much the same. Both interviews lasted around 45mins but they did not feel nearly that long.
The chemistry interview began with a fairly simple mathematical question on chemical calculations (I was allowed to use a calculator which was provided) this seemed to serve as a warm up for the later questions. The rest of the questions were on organics. Some involved concepts I had not yet covered but I simply pointed this out to the interviewers and they walked me through the basics before proceeding.
The physics interview was more varied with some pure maths and graph sketching as well as open-ended experiment based questions (for which I presented like 25 wrong solutions). These questions were harder than the ones I faced in chemistry. I often required hints from the interviewer to get to a correct answer. I remember being presented with a list of ~6 functions and the interviewer picking one out for me to sketch but he chose the easiest looking one and I was terrified that I had already failed and he was taking pity by giving me an easy question. This (fortunately) turned out not to be the case.
Both interviews consisted entirely of scientific questions, no mention of personal statements or other interests.
I prepared for the pre-interview assessment by doing the past papers and those from Oxford. I had
Try and be as enthusiastic as possible for your interview, leave a little silence as possible, give suggestions even if your 90% sure they're wrong and maybe ask them a few questions, even if you're a naturally shy and quiet person (as I am). Do you best, and try to leave the panic and uncertainty for afterwards where it can't affect your chances.