What if Things go Wrong?
The first thing to remember is that "bad" class meetings almost always feel worse to the instructor than to the students. I think we have all given really bad lectures and then discovered that the students barely noticed. So, even if the case discussion doesn't go as you had planned, things probably weren't as bad as you think.
The most common concern new case teachers have is that the method makes it hard to control what students hear, so that it seems quite possible for the class to be misled or derailed in a way that leaves students drawing "wrong" conclusions, applying methods incorrectly or failing to deal with important issues in the case. If students appear to be headed in the wrong direction, and questioning doesn't lead them back, it is perfectly okay to step outside your role as guide and back into your role as authority, and give a short lecture or some explicit correction to what's going wrong in the discussion.
Another big fear of new case teachers is too much silence. Be sure to give students plenty of time to process your question and formulate an answer before repeating, rephrasing, or answering the question yourself. What seems like an eternity to the instructor is actually very little time. If waiting and rephrasing don't work, it may be time to shake things up by breaking students up into small groups, bringing in some role play, or asking students to pose questions about points in the case they do not understand.
Finally, expect your students to fail at times. Failure is the flip side of solving problems. Be prepared to redirect your wrap-up discussion to address failures. Explain the problem and the expected answer, and ask your students to brainstorm to figure out why they didn't find that answer.