To support productive struggle, prospective secondary mathematics teachers (PTs) need to elicit and respond to students' mathematical ideas in ways that focus on those ideas and that position students to build on those ideas. Using the Teacher Response Coding framework (Van Zoest et al., 2021) we analyzed PTs' responses in three rehearsals of instruction. We identified significant differences in the natures of responses in one rehearsal compared to the other two. Using a Levels of Constraint framework based on findings of Kavanagh et al. (2020) we compared structures of the rehearsals and developed hypotheses regarding which aspects of structure might account for differences in PTs' responses. [For the complete proceedings, see ED630060.]