Although many of our interactions with technology nowadays take place in public places (e.g., using a mobile phone in public transportation), research and design on Human-Computer Interaction (HCI) has paid little attention to how this kind of technology usage affects others present—and vice versa. To illustrate the perspective of the attendant, i.e., a person who is not interacting with technology themselves but co-experiencing it as listener or viewer, we developed the so-called Attendant Card Set (ACS). In two studies, an expert survey and a student workshop, we tested its practical applicability and usefulness. It showed not only that experts assess the cards positively, i.e., helpful, informative, and relevant, but also that the cards can be used with laypersons for perspective-taking, creative ideation, and discussions. Thus, analyzing and/or comparing the experience of different types with the help of the ACS provides a unique approach to the consideration of the attendant perspective in the research and development process. Limitations of the present research and opportunities for future tool applications are discussed. In addition to establishing this concept in HCI, we also see potential in the transferability to other areas and contexts such as the design of public space or non-technological products.