IMPORTANCE A critical balance is sought between faculty supervision, appropriate resident autonomy, and patient safety in the operating room. Variability in the release of supervision during surgery represents a potential safety hazard to patients. A better understanding of intraoperative faculty-resident interactions is needed to determine what factors influence entrustment. OBJECTIVE To assess faculty and resident intraoperative entrustment behaviors and to determine whether faculty behaviors drive resident entrustability in the operating room. DESIGN, SETTING, AND PARTICIPANTS This observational study was conducted from September 1, 2015, to August 31, 2016, at Michigan Medicine, the University of Michigan’s health care system. Two surgical residents, 1 medical student, 2 behavioral research scientists, and 1 surgical faculty member observed surgical intraoperative interactions between faculty and residents in 117 cases involving 28 faculty and 35 residents and rated entrustment behaviors. Without intervening in the interaction, 1 or 2 researchers observed each case and noted behaviors, verbal and nonverbal communication, and interaction processes. Immediately after the case, observers completed an assessment using OpTrust, a validated tool designed to assess progressive entrustment in the operating room. Purposeful sampling was used to generate variation in type of operation, case difficulty, faculty-resident pairings, faculty experience, and resident training level. MAIN OUTCOMES AND MEASURES Observer results in the form of entrustability scores (range, 1-4, with 4 indicating full entrustability) were compared with resident- and faculty-reported measures. Difficulty of operation was rated on a scale of 1 to 3 (higher scores indicate greater difficulty). Path analysis was used to explore direct and indirect effects of the predictors. Associations between resident entrustability and observation duration, observation month, and faculty entrustment scores were assessed by pairwise Pearson correlation coefficients. RESULTS Twenty-eight faculty and 35 residents were observed across 117 surgical cases from 4 surgical specialties. Cases observed by postgraduate year (PGY) of residents were distributed as follows: PGY-1, 21 (18%); 2, 15 (13%); 3, 17 (15%); 4, 27 (23%); 5, 28 (24%); and 6, 9 (8%). Case difficulty was evenly distributed: 36 (33%) were rated easy/straightforward; 43 (40%), moderately difficult; and 29 (27%), very difficult by attending physicians. Path analysis showed that the association of PGY with resident entrustability was mediated by faculty entrustment (0.23 [.03]; P < .001). At the univariate level, case difficulty (mean [SD] resident entrustability score range, 1.97 [0.75] for easy/straightforward cases to 2.59 [0.82] for very difficult cases; F = 6.69; P = .01), PGY (range, 1.31 [0.28] for PGY-1 to 3.16 [0.54] for PGY-6; F = 22.85; P < .001), and faculty entrustment (2.27 [0.79]; R2 = 0.91; P < .001) were significantly associated with resident entrustability. Mean (SD) resident entrustability scores were highest for very difficult cases (2.59 [0.82]) and PGY-6 (3.16 [0.54]). CONCLUSIONS AND RELEVANCE Faculty entrustment behaviors may be the primary drivers of resident entrustability. Faculty entrustment is a feature of faculty surgeons’ teaching style and could be amenable to faculty development efforts.