At large universities, Graduate Teaching Assistants (GTAs) are a critical part of undergraduate instruction. Investigating effective and innovative ways to prepare GTAs for college level STEM instruction is an investment in undergraduate student learning and a strategic way to broadly implement active learning. Simulated classroom environments are a powerful training enhancement because they can provide a low-risk rehearsal opportunity for GTAs to practice evidence-based teaching skills before trying them on actual students in their classroom. Over the past two years, we have used a mixed-reality teaching simulator to coach GTAs in chemistry, physics, and mathematics on skills including calling on students & responding to incorrect answers, asking questions, and facilitating group work. We have found measurable impacts of GTA participation in the simulator including an increase in student-centered instruction after four simulator sessions. This workshop is an introduction to TeachLivE, a mixed-reality teaching simulator that can be used to support GTAs who facilitate active learning.