Ask questions but don't get in the physician's way either. It all depends on how good your mentor is at teaching. Unfortunately not all physicians are good instructors. Some will hold informal and very informative lectures on the side and in between patient interactions and others will act like you're not even really there and just go on with their typical day. If you have the former, you're lucky and you'll learn a lot, and if you have the latter, you may not find it to be very rewarding.

I say ask questions whenever you find something interesting or want to know more about it but at the same time realize that they're probably very busy and don't have time to explain every little thing in detail. But just be a sponge and absorb all you can. This experience is more about you deciding if medicine in every way that you've built it up to be in your mind is really what its like in real life.

So just have fun and learn as much as you can!