Secondaries or interviews?
For secondaries, you can find all the prompts on the previous year's school-specific threads. (They don't usually change much, if at all, year to year).
If we're talking about interviews, I prepared differently for "normal" interviews than MMIs. For "normal" interviews I did a lot of research into the school and the area, mocked up some bullet points for "why medicine" and "why this school", and prepared some questions in advance (and tried to add some during the day as things came up on the tour/during info sessions). I also got a list of "possible interview questions" from my premed advising office, and went through that list before the cycle started just thinking through how I would answer them.
For MMIs I read through the WashU material briefly, worked through many practice scenarios on paper to get a feel for the types of questions, and (most important!) did a bunch of practice questions timed/out loud/mock interview style. The scenarios are designed to not need any particular knowledge, usually, so the biggest hurdle for me was getting used to the format so that I could deliver something coherent and natural.
Be careful of overpreparing so much that your responses sound "canned"; obviously also be wary of underpreparing so you aren't caught off guard. It's harder the first couple times and gets easier and more comfortable as you do more.
