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 interviews: a couple of hours
Length of first interview: around half an hour; Length of second interview: around half an hour
The first interview was for going over the questions in the maths and science test - there were too many questions to finish in the time so we went through some together or tried some again I got stuck on. This ranged from Newtonian mechanics to algebra to calculus and graph sketching.
The second interview just had the interviewer asking me questions directly and once he had seen enough he would stop me and start a new question. This had some materials, integration and circuits questions. We spoke about my interest in the subject too, I think to settle me in.
I just went through my A Level knowledge and did some practice questions on i-want-to-study-engineering.org. This forms a good basis as you know what kind of questions to expect and also some tricks for integration etc.
I found talking to other students during the waiting time quite intimidating as I came from a comprehensive school background and
My main advice would be to take the mindset that the interviewer is there to guide you and get the most out of you, and you can enjoy the learning experience.