[This paper is part of the Focused Collection on Instructional labs: Improving traditions and new directions.] This study investigated the development of four new novice graduate teaching assistants (GTAs) and one nontenure track (NTT) faculty member assigned to teach an introductory physics laboratory course based on the investigative science learning environment (ISLE) approach. We describe the training that these instructors received and the progress that they made over their first semester of instruction. We used classroom observations scored using the Danielson Framework and the Reform Teaching Observation Protocol (RTOP) to assess each instructor's teaching multiple times throughout the semester. All four GTAs demonstrated improvement of various degrees while the NTT did not. Classroom observations and debriefs improved the GTAs' teaching and were used to tailor weekly training meetings to their needs. Our results suggest that new instructor training should feature specific, tangible teaching actions which new instructors can implement easily in their own classrooms. When these actions directly address issues that instructors have had, as indicated by conversations with the instructors and classroom observations, they were more likely to be adopted and have a noticeable effect on the quality of teaching.