The first officer aboard DS9 would have been Ro Laren, but she was replaced by Kira Nerys (Nana Visitor) because Michelle Forbes did not want to commit to a six-year contract working on DS9. Indeed, the reason the producers had decided to set the show on Bajor in the first place was because of Ro.
Miles O'Brien was brought aboard DS9 in this episode and made a part of the senior staff because the producers felt that Colm Meaney was too talented an actor to confine his character to a transporter room. Additionally, they hoped the TNG crossover would help boost the new series' ratings, especially since TNG's Ro Laren character, intended to be the station's Bajoran first officer, was declined by the actress who played her (her character was replaced by Kira Nerys).
The "Battle of Wolf 359" prologue takes place concurrently with one of Star Trek: The Next Generation's most famous episodes, the 2-part Borg feature "The Best of Both Worlds". That 2-parter aired in 1990, three years before this production "Emissary." The Battle scenes in "Emissary" are based on a subplot written for "The Best of Both Worlds" when it was planned to have 3 parts rather than just 2, but were not filmed in 1990 due to budget constraints. Thus this "Emissary" is the first time when the much-discussed Battle was finally shown to viewers rather than simply talked about.
Jadzia Dax and Trills seen after this episode are distinctly different from the Trills in the TNG episode "The Host." There is considerably more depth to the relationship between the host and symbiont in Trill joining than described in the TNG episode. There is no longer a visible abdomen bulge where the symbiont resides, and the fact that Trills now have spots rather than prosthetic make-up is because the producers felt Terry Farrell was too attractive to cover her face up.
The wormhole effect contains water dripping in a pan.