This paper draws from literature and our experience of conducting Wizard-of-Oz (WoZ) studies using natural language, conversational user interfaces (CUIs) in the automotive domain. These studies have revealed positive effects of using in-vehicle CUIs on issues such as: cognitive demand/workload, passive task-related fatigue, trust, acceptance and environment engagement. A nascent set of human-centred design guidelines that have emerged is presented. These are based on the analysis of users' behaviour and the positive benefits observed, and aim to make interactions with an in-vehicle agent interlocutor safe, effective, engaging and enjoyable, while confirming with users' expectations. The guidelines can be used to inform the design of future in-vehicle CUIs or applied experimentally using WoZ methodology, and will be evaluated and refined in ongoing work.