A hallmark of improvisation is the dynamic assembly of action or interaction patterns, which confers an ability to react flexibly, to be creative on-the-fly, and to make the most of the external situation. In this chapter I will argue that there is a need to elucidate the theoretical foundations of the action dimension in improvisation, the specific capability of improvisers to create a wide variety of actions and tailor them to the situation. The notion of “soft assembly”, which has roots in complexity-informed motor control theories, offers a tool for this. Soft assembly refers to a temporary functional integration of several action components that are combined as the task and context require it. I will presently try to illustrate how the mechanisms of soft assembly respond to the key criteria of improvisation: immediacy, ecological fit, and creativity.