Human Robot Interaction is entering into the interesting phase where the relationship with arobot is envisioned more as one of companionship with the human partner than a mere master-slave relationship. For this to become a reality, the robot needs to understand human behaviorand not only react appropriately but also be socially proactive. A Companion Robot will alsoneed to collaborate with the human in their daily life and require a reasoning mechanism tohandle ambiguity surrounding human engagement and their intention to collaborate. In thiswork, we will identify key elements of such interaction in the context of a collaborative jointtasks, with special focus on how the human successfully collaborate to achieve a joint action.We will the show application of these elements in a robotic system to enrich its social humanrobot interaction aspect of decision making. In this respect, we provide a contribution tothe managing robot high-level goals and proactive behavior and a description of a coactivitydecision model for collaborative human robot task. Also, a HRI user study demonstratesthe importance of timing a verbal communication in a proactive human robot physical jointaction.