David Zabel served as the series' head writer and executive producer in its later seasons. He initially joined the crew for the eighth season and became an executive producer and showrunner for the twelfth season onward. Zabel was the series' most frequent writer, contributing to 41 episodes. He also made his directing debut on the series. Christopher Chulack was the series' most frequent director and worked as a producer on all 15 seasons. He became an executive producer in the fourth season but occasionally scaled back his involvement in later years to focus on other projects.