Policymakers and educators alike increasing focus on faculty adoption of interactive teaching techniques as a way to improve undergraduate education. Yet, little empirical research exists that examines the processes whereby faculty make decisions about curriculum design and classroom teaching in real-world situations. In this study, I use the idea of the "problem space" from cognitive science to foreground the importance of actor-situation dynamics in shaping instructional decision making. Using data from a freelisting exercise and retrospective recall interviews, I describe the perceived affordances, planning strategies, and both the planned and enacted curricular artifacts for a group of 58 STEM faculty. Then, to examine these processes as they unfold in practice, I examine the cases of two physics instructors as they planned and taught specific classes. Results indicate the primacy of fixed affordances such as time, course content, and class size, and the influence of time constraints that lead to the dominant planning strategy of recycling and fine-tuning old lecture notes and PowerPoint slides. The paper also delves into the importance of these artifacts as cultural tools that mediate practice. The paper contributes a new approach to the study of instructional decision making while also providing actionable insights to improve practice. In particular, I recommend instituting departmental policies that require faculty to engage in brief, post-class and post-course reflection that results in minor updates to their curricular artifacts and will not require a significant time commitment.