Just bear in mind that they are looking for students who have a critical mindset. Always take a moment to digest and have a quick think about the question that has being asked and don't be afraid to ask them to clarify something if you are unfamiliar with a word they have used or need some context. They will not be expecting you to be an expert on the subject you have applied for as they will teach you that, but they do expect you to be familiar with the subject to the level of any qualifications you already have or are currently studying for. They want to see how you go about thinking and constructing your answer, so it is good to talk out loud about what you are considering as factors in your answer before giving an actual answer. They usually will keep going with questions until you struggle to answer something, that is normal so just because you struggle on a question is not necessarily an issue, it's how you go about answering it. Familiarise yourself with your personal statement and be prepared to answer questions on that too.