Regarding "discussion sections" for undergraduate science courses I will be undertaking my first teaching position as a graduate student in the fall - I will be leading two of the discussion sections for an undergraduate biology course.

What sorts of things should I expect to teach in a discussion section for a science course and how should I teach them? 

When I was an undergraduate and took the same course (same material, different university), the graduate students had to re-teach the lectures in our equivalent of discussion sections, as if it was brand new material because the professor was not a very good lecturer.  I know the professors for whom I will be teaching these sections are rather good lecturers, so I do not expect to have to re-teach everything to the same extent.  I have several concerns, though:


These sections are about twice the size of those at my undergraduate university, and I would like to encourage regular class participation of some sort, to make sure students stay engaged.
The material is undoubtedly challenging so I am wary of bringing in too much new material, such as discussion of scientific papers, which is the first kind of discussion that springs to mind.  (Also, someone will inevitably ask "is that going to be on the exam?")


Because the material is challenging, I could probably do with some review. But how to I keep a large number of students engaged (and participating) through repetition of lecture material? Ostensibly, the reason for having discussion sections is so that there is actual discussion.
I can say with 100% confidence that it will depend on the professor(s).  That being said, you should expect to:


Reteach all content.  You won't actually have to, but you should of course be able to.  Students often use discussion sections to ask questions they don't want to ask the prof for fear of "looking dumb."
Give meaningful practice problems and explanations the students should familiarize themselves with for an exam.
For some classes, actually lead a discussion of topics.
Just use it for office hours.