What do you want to hear from the students?
This seems like a simple question, but it actually takes some careful consideration. Do you want students to answer comprehension questions or do you want them to have authentic discussion? Often, teachers lead a “discussion group” and the only person talking is the teacher. They ask simple yes/no or short answer right-or-wrong questions.  This is very comfortable for teachers because they control the pace and direction of the discussion.  If you want to encourage authentic student-generated discussion, you might have to step outside of your comfort zone.

Also see posts about accountable talk discussion prompts and grouping configurations.

The Questioning Pyramid
You can think of discussion prompts as a pyramid - very broad, open-ended questions at the bottom and increasingly specific questions as you move up.

Scaffolding Questions
Questions at the base of the pyramid require more thought and lend themselves to student-directed discussion. They are also much more difficult and may result in blank stares and awkward silences. If students are not used to answering such broad questions, you can scaffold up the pyramid. Always start with the most broad, then move toward more specifics if the students get stuck. Your can always lead them back to the base as the discussion gets going.