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: Cambridge Law Test (CLT)
Number of interviews: 2
Skype interview: No
Time between interviews: 30 minutes
Length of first interview: 30 minutes; Length of second interview: 30 minutes
First there were some introductory questions to calm me down. Then they asked me pretty open (very discussable) questions which got progressively harder and more technical.
I thought the best way to prepare was to read the news from many different sources, like the BBC and The Guardian, as well as reading about legal and ethical issues online
I would reccomend reading the news, in order to be up to date with complex, debatable issues. I would also recommend reading your personal statement beforehand