They have something to say so they say it, whether it takes the theme forward or not. Sometimes what they say is a distraction sapping energy from the main flow of the dialogue. Eventually progress slows as momentum is lost and participants get tired.
A key role of the leader then is to ensure contributions from the group add to the issue at hand, whilst at the same time being open to truly relevant alternative perspectives, which just might be revelations.
How do you keep a balance? How do you coach people to be mindful of whether they are on-topic or off-topic? How do you keep your contributions relevant yourself?