Abstract Measuring attention and engagement is essential for understanding a wide range of psychological phenomena. Advances in technology have made it possible to measure real-time attention to naturalistic stimuli, providing ecologically valid insight into temporal dynamics. We developed a research protocol called Trace, which records anonymous facial landmarks, expressions, and patterns of movement associated with engagement in screen-based media. Trace runs in a standard internet browser and resembles a contemporary media player. It is embedded in the open-source package PsychoJS (the JavaScript sister library of PsychoPy) hosted via Pavlovia, and can be integrated with a wide range of behavioral research methods. Developed over multiple iterations and tested with over 200 participants in three studies, including the official broadcast of a major theatre production, Trace is a powerful, user-friendly protocol allowing behavioral researchers to capture audience attention and engagement in screen-based media as part of authentic, ecologically valid audience experiences.