Agile software teams' frequent releases and fast iterations present a growing need for rigorous user experience research methods that are faster, lighter-weight, and more flexible. To this end, we developed the Informal Cognitive Walkthrough (ICW). This agile research methodology grew organically, over the course of three years, while working with a very large agile software development team. ICWs involve conducting one or more Simplified 'Streamlined Cognitive Walkthroughs' (SSCW), followed by one or more Simplified 'Pluralistic Walkthroughs' (SPW). In this paper, we present the ICW and provide a real-world example of its application. Preliminary experiences with the method revealed potential advantages over traditional lab studies, ranging from more quickly uncovering and fixing usability issues, to a stronger collaboration between the disciplines, and to acting as a forcing function in aligning diverse engineers to deliver on a common user goal.