There are 2 main types of additional questions. The first type is the short questions, asking about a favourite book, artist, item, or whatever. Be careful what you answer, and yet be honest, since on the interview you may be asked to elaborate on some of your responses. It is also good if you can show the emotional impact this artist, book or item has had on you.
The second type of questions require you to write a whole essay. Topics may vary from "How can you contribute to our university's community?" to "Tell us about the experience and/or person that has changed you the most.". Here there are no rules and no requirements (except for writing something on topic, of course), so be original, show your way of thinking. The only thing is - as we said before - be honest and DO NOT copy other pepople's essays. Univirsities have ways to find out whether you've been honest or not.