This paper describes the use of the Gestrument Engine as a tool to develop a more articulated understanding of the interaction between user and digital instruments in musical creativity. Through the use of the Gestrument Engine, the paper seeks to develop a new model, the Musical Interactivity Area. This is a visualisation of the area between a composed piece of music and a playable instrument, mapping out possibilities that arise when using tools where it is possible to both pre-define the confines of the input data as well as the structural and temporal musical content. The Gestrument Engine is used by defining several alternative versions of the same musical rules, and later positioning these versions within the Musical Interactivity Area to exemplify the model.