Background: Pair programming is a well-established and versatile agile practice. Previous research has found it to involve far more different roles than the well-known Driver and Observer/Navigator roles. Pair programming often involves heavy knowledge transfer from mainly one partner to the other. Objective: Understand how to fill the ensuing Teacher and Student roles well (positive behavioral patterns). Understand how they may break (anti-patterns). Method: Open coding and axial coding of 17 recorded pair programming sessions with 18 developers from 5 German software companies, plus interviews with 6 different developers from 4 other German companies. Results: We describe six facets of effective Teacher behavior (e.g. Prioritizing Knowledge Transfer) and two facets of effective Student behavior (e.g. Expressing Knowledge Wants). We describe four harmful would-be-Teacher behaviors (e.g. Pushing Unwanted Knowledge), and one harmful would-be-Student behavior (Failing to Provide a Back Channel). Conclusions: The role facets can serve as learning goals and to-do list for developers who want to develop strong pair programming skill. The anti-patterns can serve as warnings for one's own general behavior and as triggers for immediate meta-discussion if they occur within a pairing session.