When Nicole de Boer read for the role in 1998, she was considered "one of the busiest actresses in Canada but a relative unknown south of the border". She used her own camera to tape her audition. DeBoer admits that prior to being called to the show she didn't know what a Trill was. But she made sure to study, and watched previous shows, though careful not "to watch a whole bunch of Jadzia stuff.
The fictional character of Ezri Dax was introduced in the first episode of season seven. The back story states that Ensign Ezri Tigan, a Trill Starfleet officer, upon leaving Starfleet Academy was assigned to the USS Destiny as assistant ship's counselor. She had no interest in becoming joined with a Trill symbiont (in the episode "Shadows and Symbols").
After Lieutenant Commander Jadzia Dax is killed aboard Deep Space Nine, the Dax symbiont is saved by Dr. Julian Bashir and sent to Trill ("Tears of the Prophets"). However, the symbiont becomes gravely ill during its journey on the Destiny and, as the only Trill on board, Tigan is asked to join with Dax in order to save its life ("Shadows and Symbols"). Ezri is Dax's ninth host ("Dax", "Facets").
Upon joining, Ezri is assigned to Deep Space Nine and promoted, because of her extensive Starfleet experience and knowledge from the Dax symbiont, to lieutenant junior grade ("Afterimage"). She serves as counselor.
Jadzia was married to Lieutenant Commander Worf, who at first refuses to speak to Ezri, convinced that Jadzia has gone on to Sto-Vo-Kor and can no longer exist through the Dax symbiont ("Afterimage"). However, the two later become friends and, briefly, lovers ("Penumbra").
In Pocket Books' non-canon DS9 Relaunch novels, Ezri remains on Deep Space Nine but moves from counseling to command, receiving a promotion to lieutenant and becoming executive officer of the USS Defiant. Following a mission on the Trill homeworld, she and Bashir end their romance but decide to remain close friends. In the upcoming Destiny trilogy Dax is captain of her own ship.
Compared to Jadzia, Ezri comes across as very insecure. Some fans derided Ezri Dax as "Ally McTrill" (in reference to the title character on Ally McBeal), and one TV Guide reviewer took a cue from the fans. Nicole de Boer, however, took this as a compliment.
Ezri Dax was the only member of the main cast not to be made into an action figure by Playmates Toys.