Clinical supervisors fulfill a dual responsibility towards patient care and learning during clinical activities. Assuming such roles in today's clinical environments may be challenging. Acute care environments present unique learning opportunities for medical trainees, as well as specific challenges. The goal of this paper was to better understand the specific contexts in which overt teaching interactions occurred in acute care environments. We conducted a naturalistic observational study based on constructivist grounded theory methodology. Using participant observation, we collected data on the teaching interactions occurring between clinical supervisors and medical trainees during 74 acute care episodes in the critical care unit of two academic centers, in Toronto, Canada. Three themes contributed to a better understanding of the conditions in which overt teaching interactions among trainees and clinical supervisors occurred during acute care episodes: "seizing emergent learning opportunities," "coming up against challenging conditions," and "creating learning momentum." Our findings illustrate how overt learning opportunities emerged from certain clinical situations and how clinical supervisors and trainees could purposefully modify unfavorable learning conditions. None of the acute care episodes encountered in the critical care environment represented ideal conditions for learning. Yet, clinical supervisors and trainees succeeded in engaging in overt teaching interactions during many episodes. The educational value of these overt teaching interactions should be further explored, as well as the impact of interventions aimed at increasing their use in acute care environments.