This paper presents Puffin, an interactive screen-score system, and the development of a piece for the system with two instrumentalists. Puffin is designed to expose the interconnections between two musicians in a performance ecology and the role of the interactive artifact and musical score in that ecology. We conducted daily interviews with the instrumentalists during the study to understand the different interconnections that emerged among the instruments, musicians, and the score. We visually represent the resulting ecology by using ARCAA, a recently developed framework designed to study performance ecologies. In light of existing literature, this study offers new perspectives on some elements of performance ecologies: how different actors play different roles and perform different actions in different contexts; how a screen-score can act as a mediator; how a piece and a system are interconnected but distinct; and how the piece has some constraints in itself. Finally, we observed how autobiographical and idiographic design can coexist.