Field supervisors are central to clinical teaching, but little is known about how their feedback informs preservice teachers' (PSTs) development. This sequential mixed methods study examines over 3,000 supervisor observation evaluations. We qualitatively code supervisor written feedback, which indicates two broad pedagogical categories and nine separate skills. We quantize these feedback codes to identify the variation in the presence of these codes across PST characteristics, and then use several modeling techniques to indicate that specific feedback codes are negatively associated with evaluation score. Managing student attention was most detrimental to scores in early observations, whereas instructional feedback (e.g., lesson delivery) and verbal corrections were prioritized later in clinical teaching. Findings inform teacher preparation policy on understanding PST development and improving supervisory feedback.