From my experience, the most important thing is keeping up a conversation with the interviewer. The best thing is to keep your answers to about 1:30 minutes, and then wait for follow up questions or for them to move on, rather than putting them to sleep by talking for 7 minutes straight! This was something I was worried about before my MMI, but keeping the answers within a suitable time limit makes sure that they stay interested!
Again, it’s important to keep eye contact, to smile etc. so you can tel where the interview is heading. This will give you a good indication on when to wind up your answer.