In recent years, Conversational Analysis (CA) has seen an increasing interest in longitudinal studies (Deppermann & Pekarek Doehler, 2021). The recurrent experience of interactional practices leads interactants to develop routines that may sediment into entrenched patterns over time (Dreyer, 2022). Longitudinal CA thus aims to track the emergence and sedimentation of interactional practices over time. In this contribution, we analyse interactional practices of participation in play-situations. Participation in joint activities is a universal form of human sociality (Goodwin & Goodwin, 2005). Here, we focus on how children perform their own agentive participant's work and how mothers support their children in doing so. Investigating a longitudinal data set of mother-child play-interactions at ages 1 and 5, we ask whether participation patterns emerge as early as in the first year and are sedimented in the fifth year, as well as whether the synchronisation of embodied action provides crucial resources for the achievement of active participation in joint activities.