In interaction, humans align and effortlessly create common ground in communication, allowing efficient collabora- tion in widely diverse contexts. Robots are still far away from being able to adapt in such a flexible manner with non-expert humans to complete collaborative tasks. Challenges include the capability to understand unknown feedback or guidance signals, to make sense of what they refer to depending on their timing and context, and to agree on how to organize the interaction into roles and turns. As a first step in approaching this issue, we investigate here the processes used by humans to negotiate a protocol of interaction when they do not already share one. We introduce a new experimental setup, where two humans have to collaborate to solve a task. The channels of communication they can use are constrained and force them to invent and agree on a shared interaction protocol in order to solve the task. These constraints allow us to analyze how a communication protocol is progressively established through the interplay and history of individual actions. We report preliminary results obtained from a pilot study, and discuss how the understanding of strategies used by humans could be useful to achieve more flexible HRI.