There is an assumption that graduate teaching assistants (GTAs) have mastery of the chemistry content they are teaching and preparation to instruct students. However, in reformed classrooms, GTAs may need more support and resources. We designed chemistry rehearsal modules for use in a mixed-reality teaching simulator to help prepare GTAs for instruction in reformed classrooms by targeting pedagogical skills. The modules included familiar curricular activities and research-based student conceptions. We conducted an exploratory study with three experienced GTAs to investigate the utilization of the module inside the simulator. The three GTAs practiced cold calling and error framing and participated in a 1-h interview based on their experience. We recorded, transcribed, and coded the interviews and the simulator sessions. The three GTAs implemented the skills and provided feedback that supported the use of the module in the simulator and the authentic nature of the practice. Simulated environments that encourage personalized learning, skill practice, and responding to literature-based student ideas provide a promising avenue for graduate teaching assistant professional development.