Undergraduate laboratory courses can provide opportunities for students to participate in science practices. This requires rethinking both curricula and instruction. Science practice--based courses require students to be positioned as epistemic agents, implying a shift in instructor role. Teaching assistants (TAs) are the primary instructors for laboratory courses. The current study aims to understand how TAs support students in science practices. Specifically, we sought to characterize variation in teaching and to understand how TAs learned and adapted their teaching approaches over time. Our study takes place in the context of a large, introductory laboratory course, Authentic Inquiry through Modeling in Biology (AIB-Bio). Our approach investigated the intersection between instructor reasoning and actions using stimulated-recall interviews, where instructors reflected on audio recordings from their classrooms. Application of our conceptual framework revealed that TAs' instructional roles and purposes were fluid and influenced how they supported students' science practices. We also showed how interactions with students cued fluctuations in TAs instructional approaches. Results include a case study that suggests potential mechanisms for TA learning. We propose a model to explain the variation in the enactment of a science practice--based curricula. We end with practical implications to consider when building professional development for science practice--based instruction.