Agency has been identified as a personal asset for teachers that may enable them to competently manage their classrooms (Stein, Kintz, & Miness, 2016). We explored how tools from systemic functional linguistics (Halliday & Matthiessen, 1994) can be used to track the agency instructors express. We interviewed graduate student teaching assistants (GTAs) and professors from mathematics departments to track their articulation of agency regarding responsibilities associated with interpersonal, disciplinary, institutional, and individual obligations of the role of mathematics instructor (Chazan, Herbst, & Clark, 2016). It is informative to compare these two groups because, while they all teach critical introductory mathematics courses, they also have recognizable differences in status. We report that while GTAs present themselves as similarly agentic as professors in relation to their disciplinary obligation (to the integrity of the mathematics they teach), they vary in relation to their other obligations. We draw implications for the professional development of GTAs.