Communication channels physically constrain the flow and shape of human language just as irresistably as a river bed directs the river’s current. Escarpments speed the current, sculpting jetties and whirlpools. Meadows encourage evenness, a certain recumbency. The flow becomes a deafening cascade as it passes over granite boulders, and is arrested abruptly behind man-made dams. In short, the river is molded, rendered navigable or not, through the physical medium of its own bed. Although the communication modalities may be less visually compelling than the terrain surrounding a river, it is a mistake to assume that they are any less influential in shaping the language transmitted within them. Understanding the influence of communication modalities begins with an identification of their landmark features, and of the observable impact of these features on language. One goal of this chapter is to provide a comparison of the discourse and performance characteristics of instructions presented in three different modalities, each of which was classified according to the presence or absence of: 1) speech and 2) interaction. A second goal is to begin constructing an analytical framework from which predictions can be made about the separate impact of speech and interaction on specific aspects of discourse and performance.