Hey Claire!
I'm afraid I'll be useless in the vet med area, but I can say from previous interview experiences that once you show your reasons (usually incorporating your passion, potential/ aptitude and suitability for vet med) and speak with clarity and confidence, things can't go too wrong. Don't try to overthink or over-correct your answers - Occam's razor is often the best motto when you're stuck on what they're looking for! Experience with volunteering at animal shelters or related experience, whether that be from owning a pet yourself or helping your grandma on the family farm, will also bode well. If all else fails, go in with guns blazing and make sure to give it your all! Nothing can stop them from saying your didn't try your best, if you really did!
I'd probably prepare by actually writing down your reasons in words on paper. This way, you can see them properly and change them to your liking. Just ensure you don't prepare a full script; this comes off as over-rehearsed and your delivery falls flat. Instead, dot point your motivations and previous experience. The reasons of why you want this is almost guaranteed to be asked and people stumble on it way too easily.
Hope that helps you.