Denis Crosby came up with the idea for Sela during her time on Star Trek: The Next Generation.
Denise Crosby rose to fame as a member of the U.S.S. Enterprise-D during the first season of Star Trek: The Next Generation. Crosby, originally up for the role of Deanna Troi, but ultimately got the role of Tasha Yar, the ship’s chief security officer. During her first season, Crosby grew disenfranchised with the show and requested to be killed off.
The request was granted and Crosby left the show. She’d reprise the role of Yar in season three’s episode, “Yesterday’s Enterprise”, which marked the 15th episode of the season. It was an episode based in an alternate timeline where the Federation was more militarized than before, and Yar was still alive.
It also marked the debut of the U.S.S. Enterprise-C, the predecessor to the Enterprise-D, the ship used in The Next Generation. The ship was out of its time, so to speak, and both crews had to send the ship back to where it was from in order to reset the timeline to its proper state. Even though in doing so, the Enterprise-C would be destroyed by awaiting Romulans.
Crosby apparently had a great idea during this shoot and came up with the idea of the half-Romulan, half-human Sela.
Denise Crosby set in motion one of Star Trek: The Next Generation’s best villains.
According to The Hollywood Reporter, Crosby was the one who came up with the idea that she returns to the show as Sela, daughter of Yar and a Romulan who capture Yar after she returned in time to aid the Enterprise-C.
"Crosby had the idea for Sela during production on the classic season three episode “Yesterday’s Enterprise,” and Moore and the rest of the TNG writing staff used the events leading up to “Redemption” as a way to introduce fans to that character."
Sela would debut during the end of season four, on a pivotal cliffhanger that involved the Klingons and Worf.
The move was a genius one, as Sela would become one of the best villains on the Next Generation. The role also helped Crosby with her decision to return, as she was originally bored by what Yar was given. Sela gave Crosby a brand new character to play and allowed her to stretch acting muscles she felt she wasn’t getting with Yar.
In the end, it seems rather interesting that an actress who quit the show would end up being the one to come up with a season-finale game-changing character like Sela.