Despite clear links to improving college outcomes, descriptive details about the kinds of student-faculty interactions that are most helpful for STEM students are less well understood. In this qualitative interview study, we investigate what micro-interactions are most helpful – and consequential – for STEM students’ persistence and success, especially transfer students. We found that students who experienced positive, caring interactions with at least one faculty member in their major tended to feel more connected to their discipline, had greater confidence in their abilities in STEM, and often put forth more effort in their studies. Transfer students were especially impacted by the quality of care they received from faculty; those who had negative experiences with faculty were often left feeling less capable and discouraged to continue in their STEM discipline. STEM faculty should show interest in students’ learning and long-term academic and career goals and provide sufficient availability for one-on-one help.