The scriptures should speak for themselves. The participants can often add color and meaning by stating what the scripture says in their own words, and how it relates to their lives. The leader can stimulate discussion by asking questions, reading or having the scripture read, and then stating or asking someone to state the answer. Preaching a sermon and closing the service and leaving, is certainly not an effective way of teaching. Not everyone grasps the contents of the message immediately.
Discussion often helps to clarify what was said.
Paul's long sermon at Troas led to Eutychus' falling asleep and tumbling out of the window. Lack of preparation leads to lack of learning. Participants who lead the discussions off on tangents can disrupt and confuse the others. Argumentation has no place in worship services. Love does not argue and seek to have its own way.