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: BMAT
Number of interviews: 3
Skype interview: No
Interview spread: all in one day
Length of interview: 30 minutes
There was some personal statement discussion, with a more in-depth analysis of some of the subjects I mentioned that were of interest to the researchers. Generally 3 interviewers were present, who would each present one problem, which they would question you to go through, asking you to reconsider answers, etc. There was some document analysis: for example, 'What could you conclude from this graph?'
The interview was not stressful beyond measure, the interviewers were welcoming and not so intimidating, just insistent in making sure you justified all answers.
I did practice papers from the official website in timed conditions; practised mental maths through any method available; and re-read my relevant (biology, chemistry, physics, maths) notes from high-school
For interview preparation, I did a lot of extra reading: scientific papers, books, textbooks, podcasts (not as cramming for interview, but starting earlier if possible). I did one
What I would do differently: Start revising for the pre-admissions tests earlier and don't try to revise specific material, rather stay informed about recent developments in your field