A role description for the facilitator that has been approved by the group. Be compassionate by being flexible not rigid, positive not punitive, inquisitive not judgmental, transparent and human, rather than cryptic and mechanical. Intervene for the well-being and progress of the group. Intervene more frequently at the outset to set precedent for behaviors specified by the agreements. Later, don't intervene on every infraction.
Effectiveness requires both alertness and flexibility.
Instead of naming infractions, suggest a relevant alternative that is in keeping with the agreements. If someone is taking a lot of air time, you might ask if those who have not spoken would like a chance to speak. Be genuinely curious and exploratory about group processes and needs. If there is a dilemma about how to proceed, name the dilemma and open a time-limited conversation about options. Be transparent and human.