This depends on who the service puts you with. Hopefully you will ride with a service that has intelligent management who will put you with a preceptor or atleast a medic who likes to teach. If so, while on call, keep your mout shut and observe, make a short list of questions to ask after the call. If you are with a sorry service, I wouldnt ask alot of questions. You will probably be smarter (bookwise) than their old goats who havent opened a book in ten years, thus they will be intimidated by you. Realize going in, that the book way is not usually the streetway (we can argue which is right or wrong all night), so when you ask questions do not ask as if you have noted something wrong, just ask in a general way (example: I found it interesting that you administered calcium chloride to that cardiac arrest patient, what symptoms led you to that treatment choice? versus I just finished taking ACLS, and my teacher says that Calcium isnt used anymore, why are you assholes behind the times ?).
I dont think you should have to buy lunch or do extra chores, it is nice for you to volunteer if you wish. Good Luck !