They do their own thing and try to facilitate learning. However, in PBL the instructor's role is to facilitate. The instructor is a coach. The coach tries to bring out the best in the group. These reflective skills are part of effective problem solving and group skills.
Monitoring is a key component in effective problem solving.
Trust is nurtured by such an environment. The facilitator is not the group's expert resource who will provide the answers nor should the facilitator use this as a chance to lecture. One might think that the best facilitator would be a non-expert in the discipline subject under consideration. However, such a facilitator is not sure when the students are off-base or if they have misinterpreted the information. This is detrimental to the student's learning and to the facilitator's morale.