  Spoken dialogue systems (SDSs) have been separately developed under two
different categories, task-oriented and chit-chat. The former focuses on
achieving functional goals and the latter aims at creating engaging social
conversations without special goals. Creating a unified conversational model
that can engage in both chit-chat and task-oriented dialogue is a promising
research topic in recent years. However, the potential ``initiative'' that
occurs when there is a change between dialogue modes in one dialogue has rarely
been explored. In this work, we investigate two kinds of dialogue scenarios,
one starts from chit-chat implicitly involving task-related topics and finally
switching to task-oriented requests; the other starts from task-oriented
interaction and eventually changes to casual chat after all requested
information is provided. We contribute two efficient prompt models which can
proactively generate a transition sentence to trigger system-initiated
transitions in a unified dialogue model. One is a discrete prompt model trained
with two discrete tokens, the other one is a continuous prompt model using
continuous prompt embeddings automatically generated by a classifier. We
furthermore show that the continuous prompt model can also be used to guide the
proactive transitions between particular domains in a multi-domain
task-oriented setting.
