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: 1 day
Length of interviews: 30 minutes each
First interview was inorganic chemistry - we talked about polymers and type of inorganic compound and sketched some graphs.
Second interview was organic chemistry and a question on pure maths and a type of mechanism
(1) know A level chemistry, maths and physics inside out (2) read beyond the syllabus (3) practise graph sketching and calculus (4) do past chemistry olympiad/c3l6 papers