To develop a comprehensive teacher observation instrument, 56 school practitioners submitted 200 critical incidents--descriptions and judgments of their observations of teacher-pupil interactions in the classroom. Five teacher roles emerged: instructional facilitator; instructional manager; instructional evaluator; instructional designer; and diagnostician. Because the instrument was designed to observe student-teacher interaction, it focused on the first two roles. The instrument recorded five context variables: time of day or week; subject matter; lesson type--introduction, review, or conclusion; size of instructional group; and teaching style--open plan vs. traditional classroom. A pilot study showed a distinct difference in teaching behavior patterns between open and traditional classrooms. (A sample data sheet for recording teacher behavior is appended.) (CP)