Discussions are difficult, both for teacher and for students
It might help you, at first, to think of discussion as being entirely different from other kinds of activities that are common in your classroom. If you think of discussion as being both new and different for students, you are far more likely to explain what you want from the students, to give them reasons for wanting it and to teach them to do their part in the activity.
You are far less likely to assume that students will know what to do. When a teacher is standing silent in front of the room and students are sitting in their seats, one cannot tell whether the teacher intends to begin:
 a lecture (in which students are to listen actively),
 a demonstration (in which students should watch closely what the teacher does with an object),
 a recitation (in which students are to respond when called upon), or
 a discussion (in which students are to take initiative and participate actively).
If a teacher asks a question, the students still cannot know what's expected because we often use rhetorical questions in lectures, where we expect students to think about the question, not offer responses to it. We often ask a question to start a recitation, where we will expect students to give valid answers so we can give them immediate feedback. That's not discussion either.
Teachers always depend on students for their own success. That is acutely the case for discussions, where students must take initiative and act as your partners.
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