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
Time between each interview: 15 minutes
Length of interviews: 15 minutes each
I was given a source and asked to compare two different ancient societies described in it.
In the second interview, I was given a picture of an archaeological dig and asked to analyse it.