Most undergrads are timid in class because they don't want to look foolish in front of their peers. You have to help keep that from happening if you want to encourage a discussion. Give a challenging assignment at the beginning of the quarter. Those students who really need or want the course will not leave. It is easier to start out tough and lighten up later than to do the opposite.
Grades can have such a big effect on students' fortunes.
In our experience, there is little question about who deserves the As and the Fs. It's everything in between that is difficult. Grade and return assignments quickly. Quick feedback is more effective than slow. It shows the students that you care about them.