"Lectures" in Physics 131-132 are different from other large lecture courses you may have experienced.  Rather than sitting and listening, the students are engaged in group discussions and problem-solving for a significant portion of the lecture time.

Often, this takes the form of Peer Instruction:  the class is given a conceptual multiple-choice question that probes their understanding.  Students answer with electronic clickers, and then everyone can see a histogram of the class's responses, which may show deep disagreement.  Next, the students turn to the people around them and discuss their reasoning to try to convince each other, and then vote on their clickers again.  Sometimes these group discussions have led the class to a new consensus; other times, the divisions remain, and this process may continue for multiple rounds or there may be a whole-class discussion.

The students will have whiteboards and markers which they can use to collaborate with their peers.

As a TA or LA who will be circulating around the room during lectures, your role is twofold:

1) Help students make the most of active learning.

• Encourage participation.  Especially at the beginning of the semester, some students may be hesitant to discuss their ideas with their neighbors, either because they tend to be quiet people, or because they think (rightly or wrongly) that they already get it, or because they think (rightly or wrongly) that they're completely lost.  If you see students who aren't talking, you can start by asking them to explain their answers.
• Model scientific discourse.  You can encourage students to provide justification for their answers, and ask them whether they agree with their groupmates (and why).
• Answer student questions.  Sometimes students get stuck when they're confused about what a question is asking.  In these cases, you can help clarify things for them, so that they can proceed.  (However, it's not your job to solve the problem for them, or even to tell them how to solve it.)
• Listen.  If there are productive conversations happening, let them happen!  You don't always need to get involved.

2) Be the eyes and ears so that the course can be responsive to the students.

• Listen.  As the people on the ground, you'll gain a solid understanding of what areas are most difficult for the students and what areas they've mastered.  You might also hear the students making new and interesting connections that weren't anticipated.
• Provide feedback to the instructors.  It might happen that the class moves onto the second question and you notice that a large number of students are still confused about the first question.  This is very useful data to convey to the instructors - it means that they might have to go back and spend more time on the concepts that were confusing.