A distinguishing feature of dialogue is that contributions can be fragmentary or incomplete. Such incomplete utterances may be later completed by another interlocutor. These cross-person compound contributions (CCs) have been hypothesised to be more likely in predictable contexts but the contributions of different sources of predictability has not been systematically investigated. In this paper we present an experiment which artificially truncates genuine contributions in ongoing text-based dialogues, to investigate the effects of lexical, syntactic and pragmatic predictability of the truncation point on the likelihood of one's interlocutor supplying a continuation. We show that what is critical is the actual and presumed accessibility of common ground, and that while people are sensitive to syntactic predictability, this alone is insufficient to prompt a completion.