Through the increased growth of speech agents, text based chatbots and social robots, language interactions with machine dialogue partners are now commonplace. Discovering what drives the way we converse with machines is fundamental to understanding our interaction with such automated dialogue partners. However, understanding of what governs user language choices in such human-machine dialogues (HMD) is sparse [14]. This Special Interest Group aims to be a catalyst for discussing and building fundamental theories of how people produce language when engaged in conversation with conversational user interfaces (CUIs). The main objective is to bring together researchers across CHI and related communities (e.g. HRI, CUI, cognitive science, linguistics and speech technology) to map the grand challenges required to be addressed to generate evidence-based theories to explain what impacts our linguistic interactions with CUIs.