A crucial aspect of MTEs’ expertise includes pivotal interventions, defined as verbal interventions by MTEs that prompt teachers to examine their mathematics for teaching. This study aimed to identify pivotal interventions made by an MTE during interactions with prospective mathematics teachers. We collected qualitative data through observation within an initial mathematics teacher program in Brazil. Grounded theory procedures inspired the analysis, identifying four types of pivotal interventions: exploring a concept, presenting a conceptual understanding, presenting an argument, and seeking mathematical connections. The findings suggest that pivotal interventions are critical in shaping prospective mathematics teacher education, highlighting the importance of MTEs’ communication patterns with teachers. Future studies may identify additional types of pivotal interventions made by MTEs.