It might be reassuring to hear that the King's Admissions organiser told us at an open day that in the interview the only things they are interested in are finding out what your relationship with the subject is, how you think and what it would be like to teach you.
She specifically said that there is no need to come in confidently, shake hands, give good eye contact and only give perfectly formed, fluent answers. If someone shuffles in, stares at the floor, doesn't manage to construct perfect sentences but expresses some interesting insights, they are equally likely to be given an offer.
Being nervous won't be a barrier to you getting an offer - I happen to know a couple of very nervous people who got offers last year and a very confident person who didn't. Those nervous people also thought that their interview hadn't gone well, where the confident person felt that it was excellent - it may simply be that the interviewers were pushing those who then got offers, and decided fairly quickly that they didn't want the confident candidate and didn't want to torture them. So - if it feels like they are leaning on you during the interview, that can only be a sign that they think there is more capacity to squeeze out.
Like any place full of smart people, Cambridge is also full of 'weird' people (and I say that with affection, counting myself as one of those weirdos, though not at Cam). Social conventions at interview aren't crucial - your interviewer may well not have a good grasp of them either!