Stillwell co-wrote the TNG episode "Yesterday's Enterprise," a fan favorite, in 1990, and briefly took a turn before the camera as a Klingon extra in 1991's Star Trek VI: The Undiscovered Country.
He co-wrote the 1995 Voyager episode "Prime Factors" with David R. George III in 1999, and collaborated with George and Armin Shimerman on the story upon which the 1999 DS9 novel The 34th Rule was based.