The physics instruction at UC Davis for life science majors takes place in a long-standing reformed large-enrollment physics course in which the discussion or laboratory instructors (primarily graduate student teaching assistants) implement the interactive-engagement (IE) elements of the course. Because so many different instructors participate in disseminating the IE course elements, we find it essential to the instructors' professional development to observe and document the student-instructor interactions within the classroom. Out of this effort, we have developed a computerized real-time instructor observation tool (RIOT) to take data of student-instructor interactions. We use the RIOT to observe 29 different instructors for 5 hours each over the course of one quarter, for a total of about 150 hours of class time, finding that the range of instructor behaviors is more extreme than previously assumed. In this paper, we introduce RIOT and describe how the variation present across 29 different instructors can provide students in the same course with significantly different course experiences.