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: Natural Sciences Admissions Assessment (NSAA)
Number of interviews: 2
Skype interview: no
Time between interviews: 30 minutes
Length of first interview: around half an hour; Length of second interview: around half an hour
All of these were pretty focused on academic science that I'd not done before. The atmosphere is very relaxed due to the professors and they're very willing to help you throughout the process. The questions all were advanced physical NatSci questions but they can you through so they can see your thought process and problem solving abilities.
Practice Papers from previous years, revising notes
I really didn't do anything special for it - just developed my school knowledge