What's the most important thing to prepare before my interviews?

I would say the most important thing is to make sure you're confident talking about anything you've mentioned in your personal statement. For humanities-based subjects at least it is important to have done a lot of reading around your subject, so if you can read more than just the books included on your personal statement it demonstrates genuine interest rather than just extra reading to bulk out your personal statement. For example, if your interviewer asks something about a particular text you mentioned and in your reply you manage to make a reference to another text which is similar (or maybe strikingly different) in a particular way, that will always impress an interviewer. However, if you have very limited time it is most important to have thorough knowledge of the texts mentioned in your personal statement, as these are the texts that you may be directly asked about. Take time to really reflect on what you've read and form particular opinions on what you appreciated about a text/what jumped out at you as an area of interest. It goes without saying that an interviewer will soon realise if you've lied on your personal statement, so if you say Shakespeare is your favourite playwright, you need to be familiar with at least a few of his works, and if you say Garcia Marquez's writing sparked an interest in Latin American culture, you need to be able to demonstrate some knowledge of Latin American culture.