I think being a bit confident but not cocky is good when coming to do interviews which is perhaps why being an all rounder to a high level in other extra curriculars could be an advantage as this can give you extra confidence.
Lots of candidates aren't confident (either socially, or intellectually) at all; in fact certain kinds of "confidence" can be positively off-putting, if they get in the way of a candidate really engaging with a question. We're much more interested in thoughtfulness than confidence. As long as they can chat with a couple of unfamiliar adults about some interesting ideas without clamming up completely, they don't need to worry about social confidence or poise.
One of the reasons we are not interested in extracurriculars is that we are highly aware that many are basically a proxy for cultural capital or money. It's nice and all that someone has two instruments to grade 8, or Gold Duke of Edinburgh, or is a competitive rower or whatever; but largely those achievements just signal that a candidate comes from a particular social and educational background where they have access to those kinds of experiences. It would be pretty unfair of us to allow those considerations into our assessment process, as many or even a majority of our applicants haven't had the chance to access expensive music lessons or high-level sport, or similar. We're also highly aware of the social confidence that can be a superficial product of certain kinds of schooling, and we aim to look beyond that.
I would say you need to be incredibly knowledgeable about your subject area and far beyond the a level curriculum imo from the questions my dc was asked.
Rather than asking for knowledge beyond the A-level curriculum (which would be, again, very unfair to candidates who don't have the financial or social resources to cover this), we aim to assess academic potential by asking quite simple or even basic questions in an unusual way, to get the applicant thinking. Most often (at least in the humanities and social sciences), there may not even be a "right" answer. We might ask a "devil's advocate" kind of question to see if a candidate can break a problem down to its basic constituents. Or give a candidate some new information they don't know, then ask them how they would revise their opinion based on this new evidence. Or ask them to use knowledge they already know from A-level to make a judgment about an unfamiliar problem. One of my colleagues in the hard sciences asks, for example, if the pitch of a wine glass goes up or down when you add more liquid to it (and actually expects the candidates to produce the wrong answer if they and their A-level knowledge is good - because the problem is not quite what they think it is!) In the humanities, I might ask a candidate who has done Tudor Reformation history at school, for example, what might be different about our understanding of history if we knew nothing at all about religion at all during the period; and if so, what other kinds of historical events from the time might then seem the most important ones to us?
I might pick out something they have said on their personal statement about what they think is interesting about their chosen subject, and ask them why they think it's true; I might then give them an opposing opinion, and ask them if it might change their view.
None of these kinds of questions require additional knowledge beyond the A-level curriculum; but they might require some thinking on the spot, or some extrapolating from existing knowledge applied to a new situation; or a willingness to challenge received ideas. I'm sure the other interviewers on this thread can think of their own examples, too - but, in essence, we're looking for how a candidate thinks. We also want to find the people with potential who haven't had the good luck to have gone to expensive schools, or have academic parents; but who are sharp thinkers and have a natural aptitude for the subject. We are very much looking at the individuals they are, how they think, and whether they would be intellectually a good fit for the course.