In designing socially interactive robotswe have focused on robot movement and its role in multi-modal human-robot communication. In this paper we describe a user-centred design and evaluation process, investigating the idea of using speed and orientation adjustments as design elements in human-robot interaction. The scenario studied includes a robotic shopping trolley that offers products to the user while both are moving in a supermarket-like environment. Our results show that if the robot slows down while making such offers, users are more prone to react upon them. However, in an early pre-study, performed only with a robot mock-up, we observed that users tended not to notice the robot's slow-down movements while offers are made, even if these movements were shown several times to them during a video-based debriefing. This phenomenon, that users react implicitly on the robot'smovements without being consciously aware of them, was confirmed during an experimental study with a fully integrated robot prototype.We discuss our results by reflecting on human-robot interaction design methods, and we propose implications from the lessons learnt in the study of the design of robot behaviours.