Are you interviewing as an adult or junior/underage volunteer? What department are you looking to be placed in? In most cases, they want to make sure you are the type of person who would maintain patient confidentiality, etc. Maturity is an important part of the interview, as is commitment -- They don't want to hire volunteers who aren't likely to show up every week or whatever time your shift is at, because volunteers are an important link in the healthcare chain. I believe they also ask questions to get a feel for what areas or dept's you are specifically interested in, if any. Previous experience with working in a healthcare setting isn't necessarily what they're looking for, but it's important that you're the type of person who would fit in and make patients comfortable.
I don't know anyone personally who has been turned down for a volunteer position at a hospital, so as long as you conduct yourself professionally, dress casually but nice, etc. I think it's a pretty easy, low-stress interview.