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.
Number of interviews: 2
Skype interview: No
Spread: One interview in the morning and one in the afternoon.
Length of interviews: 30-40 minutes each
In my first interview, we discussed my piece of submitted written work. I was in a room with two tutors and one led the discussion while the other sat making notes (which was quite strange at first but soon became OK). They probed my argument a little, but not in an impossible way and brought in other material on the same topic to see how I thought on the spot. They then showed me a source to which I responded awfully, but that didn't seem to matter. They were quite rigorous but very friendly and at the end of the day wanted a nice discussion.
Practice papers were most useful definitely and the past papers helped a lot, also generally looking at articles and sources with the same rigour you would apply in the exam. Learning to be arsey and meticulous also went a long way.
Defo don't look at the horror stories! Most interviews are not like that. The best thing you can do for yourself is try to relax because that'll mean you're thinking as clearly as you can.
Maybe a little bit of extra reading will help but mainly just know your stuff, personal statement and essay wise, and think of counter-arguments for everything you have argued.