This study investigates the teacher role in mediating the task and the learner in an advanced academic writing class. Having identified three verbal (non-)participation patterns of students in collaborative tasks (silence, dominance, and off-task talk), I examine how these interactional concerns are understood and addressed by English as a second language writing teachers. Interview findings from the writing teachers suggest that teachers play a key mediating role during the various phases of implementing a task-based lesson in order to address the concerns of silence, dominance, and off-task talk. For example, in the task design phase, the students can be assigned specific roles in their group or they can be given planning time necessary for the task. In the task performance phase, the teacher can make judicious interventions in order to encourage contributions from the quiet students or put the talkative students on hold for a while. The paper concludes with its contributions to and implications for the role of teachers in a task-based pedagogy.