Silent video tasks are currently being developed in collaboration with four mathematics teachers in different upper-secondary schools in Iceland. In silent video tasks, teachers invite students to watch a short silent mathematics film and ask them to work in pairs to prepare and record their voice-over to the video. Next, teachers listen to students’ solutions and plan a follow-up lesson in which they initiate a whole-group discussion. The data collection focused on the teachers’ expectations and experiences with the silent video task. Thus, the teachers were interviewed before and after assigning the task, and after the follow-up lesson to better understand their experiences. This paper focuses on one of the four participating teachers and discusses the potentials of silent video tasks as means of formative assessment.