Are lectures mandatory/posted online? Would any current students feel comfortable talking about how they study/volunteer/interact at Rush?
Some are mandatory and some are not. Typically, lectures are not mandatory, but workshops and discussion groups are. Lectures are posted online--you can stream it after the live lecture is over.
Personally, I go to most lectures (unless I'm super tired and feel like I have a good grasp on the material after looking over it). Many of my friends like to sleep in and spend their nights streaming (whatever fits your sleep schedule, I guess).
It's super easy and highly encouraged to get involved in volunteer activities. There is a 10 hour/year volunteer requirement--you'll find it very easy to meet that minimum as there are volunteer activities available every week (clinical (free clinics, student run clinics, health fairs, etc.) and non-clinical (hanging out with kids in the hospital, birthday parties at a school for teenage mothers, etc.).
I'm not sure what you mean by interact?? I mostly see other M1's. Sometimes M2s, as they will hold tutoring sessions, go to bar nights, and generally be around the lecture halls. There are a lot of other students as Rush (grad, OT, PT, PA, nursing, etc.)--I see them at a lot of volunteer opportunities, but don't personally have much interaction with them.
As GraMaire said, at your interview, there will be tons of people available to answer your questions! Don't be afraid to ask away!