Abstract: Although teaching artists have been conducting classroom residencies for decades, little attention has been paid to the type of professional development that prepares them for the work. Yet, as current federal education policies press for increased accountability of instructional time, the content and quality of training has become ever more important. When we see a good arts lesson, we know that a lot goes into making it effective; this case study not only describes an innovative approach to the professional development of teaching artists, but also unpacks the training processes that contribute to instructional effectiveness. A mixed methods research design was employed to examine an arts education program that adapted a partnership residency format to function as a modality for training teaching artists. The program offered mentorships, in which pairs of veteran and novice teaching artists worked with small groups of classroom teachers, grades 5-8, to design, implement and assess 15-week arts-integrated units of study. A focused study of the core training elements of mentorship, teacher collaboration and student assessment brought to light the value of these processes in raising the artists' capacities to meet high educational expectations for residencies. However, it was the deeper investigation into the behavioral dynamics between the teaching artists and their classroom teacher partners that revealed how instructional change occurred. This study is one of the few to empirically define and list these internal training processes and both quantitatively and qualitatively demonstrate their interaction and importance. The findings showed that by structuring a residency that was highly flexible yet held all participants accountable to the tasks of partnering for student achievement, the capacity of the teaching artists to host educationally effective residencies increased substantially. While the three core elements of training proved essential to this effort, the interpersonal dynamics of program participants---which were based on beliefs about the value of arts education, individual perceptions of their roles in the training program, and an ability to cooperate in a mutual learning experience---also had considerable influence on the training outcomes of the teaching artists and the learning outcomes of the students they served.