While high interactivity is one of the key characteristics of one-on-one human tutoring, a great deal of controversy surrounds the issue of whether interactivity is indeed the key feature of tutorial dialogue that impacts students' learning. In this paper we investigate three interaction hypotheses: a widely-believed monotonic interactivity hypothesis, a better supported interaction plateau hypothesis, and our tactical interaction hypothesis. The monotonic interaction hypothesis predicts that increasing interactivity causes an increase in learning; the plateau hypothesis states that increasing interactivity yields increasing learning until it hits a plateau, and further increases in interactivity do not cause noticeable increases in learning. Finally, the tactical interaction hypothesis predicts that interactivity only increases learning when interactions are guided by effective tutorial tactics. In this paper, we examine each hypothesis in the context of an empirical study, the results of which support the tactical interaction hypothesis.