Recent innovations in professional development are rife with a wide array of efforts focused on teacher collaboration. In this essay, we address some of the unexamined assumptions about the nature and significance of interactions in teacher professional collaboration, drawing on the concept of the "fourth wall" from theater and film studies. The "fourth wall" is a term used to describe the invisible wall that separates actors from their audience. We use this metaphor to interrogate the function of the fourth wall in professional learning and argue that it reflects a culture of professional learning that, despite innovations that tout teacher collaboration, upholds isolation in teaching and teacher learning and deep embedded norms of noninterference in one another's practice. We also attend to the possibilities for supporting teacher learning that breaching the fourth wall affords when shared enactments of practice are used as a context for teachers' sensemaking and collaboration.