For the most part it's one-sided. You will have a short period of time (5-6 minutes maybe) to read a prompt, and then enter the room. There the interviewer will introduce him/herself, and begin. They may give you a hypothetical situation relating to the prompt, or ask you questions about the prompt itself. And you answer.
The large majority of it is "in your opinion" and "why do you think that". They are not allowed to give you feedback, so it can be intimidating because you'll answer and they'll just stare at you! You have to be confident though, and just keep being honest with your answers. If you have a real opinion about something, share it.
The interviews were 7 minutes long I think last year? Close to that. I had extra time with all but one, so don't worry about that. It's better to answer truthfully and succinctly and sit in silence for 2 minutes, than to ramble about nothing just to fill time.
*One tip I will give is that there is always a clock in the room. When you enter the room make sure you note the time and pay attention to it! If you glance over and you have a lot of time yet, slow down on your next response and flesh it out. If you're running out of time, answer quickly.
To be honest there isn't much preparation that can be done in advance. If you were to feel like you NEEDED practice, then I would have a friend come up with a story that has some sort of moral ambiguity and print it out for you. Read it, and then have them ask you questions. "Which person do you agree with, and why?" "If this were changed in the story, would your opinion change as well?" "How do you think this relates to vet med?". The point is simply to get you to form an answer that makes sense and can be given within a reasonable amount of time. The extent to which this would help is minimal though, in all honesty.