Siddig was considered for the lead role because the producers mistakingly took his old age makeup for his real appearance. This is referenced in "Distant Voices" in which Bashir rapidly ages from a silver fox to a doddering geezer.
O'Brien finds himself 'typecast' as the hitman Falcon in Bashir's spy holoprogram. Whether or not this was intentional, it mirrors Colm Meaney's pre-Star Trektypecasting as the same ex-IRA bad guy he plays in every film (see Under Siege and Con Air).
In "The Ship", O'Brien waxes poetic about the mountains of Ireland, but another character reminds him Ireland only has hills. Colm Meany starred in the film The Englishman who Went Up a Hill (But Came Down a Mountain) about a Welsh village that insists its hill is a mountain.
Like his character, Andrew Robinson is a claustrophobe. He had trouble wearing the prosthetics early on. Garak also spoke of posing as a gardener while conducting surveillance on Romulus. His stint at the Cardassian Embassy coincided with a number of mysterious deaths that year, including a Romulan proconsul who was found poisoned. This is an in-joke by Andrew Robinson, who is passionate about gardening in real life but hasn't murdered anyone. (That we know of.)
When Kira is complaining about how she feels being pregnant with the O'Brian's baby she says "This is YOUR fault!" to Dr. Bashir. Nana Visitor was really pregnant with Alexander Siddig's child.
Aluminum Christmas Trees: "Far Beyond The Stars" seems to be a full episode about EC Comics's Judgment Day (printed around the same time this story is set), which killed the magazine when the publishers and editorial disagreed over having the hero be black.
Directed by Cast Member: Avery Brooks, Rene Auberjonois, Michael Dorn, Alexander Siddig and even guest stars Andrew Robinson and Jonathan Frakes get at least one shot behind the camera. And LeVar Burton, who appeared in The Next Generation, took a spin as well. In fact, Burton was the fifth most prolific director on the series, while Brooks and Auberjonois share sixth place.
A (barely) oblique reference was made to this on B5. In one episode, a gift shop is set up on the station. One of the characters loudly derides this idea, saying "This isn't some deep-space franchise! This station is about something!" It should be noted that the writer of this episode, Peter David, has written several Star Trek novels. (When he wrote that line into the B5 script, it was with the expectation that JMS and friends would change it, if only to a more subtle jab, before shooting; upon learning that the line was filmed and broadcast verbatim, David remarked to JMS that "...you people are dangerous over there, aren't you?")
The Deep Space Nine writers weren't above including their own subtle jabs at B5. One episode featured Bashir having to chaperone a cadre of Ambassadors visiting the station and putting up with all the crap that comes with it. According to JMS, the Writers of DS9 actually liked B5. The producers, not so much.
Andrew Robinson explicitly played Garak as bisexual or ominisexual in his first scene with Dr. Bashir. Writer Robert Hewitt-Wolfe corroborated this, adding that Bashir was supposed to be oblivious to his advances. This plot thread was swiftly bundled out of sight by Paramount, though it doesn't entirely go away (at least as far as Garak's mannerisms went.) Both characters got their own heterosexual love interests somewhere down the line.
In "Facets", Sisko willingly allows himself to be possessed by Joran Dax for a short term. Avery Brooks was required to do shoot this scene twice. The reason? Brooks' performance was way too scary for a PG show.
Kai Opaka was called "Deep Space Nun" during the first season but was Put on a Bus midway through the season.
Thanks to reviewer SF Debris, the practice of calling the Defiant the USS Ben Sisko's Motherfucking Pimp Hand, is slowly catching on.
Follow the Leader: DS9 had trouble finding its feet initially, leaning on unresolved arcs from TNG and more or less sticking to its episodic format (not unlike VOY & ENT). As Babylon 5 began to makes waves with its much-hyped, five-year arc, the Dominion War was shortly introduced. The two would remain Dueling Shows for the remainder of their respective runs, with B5 wrapping up in 1998 (a year before DS9).
Iggy Pop as a Vorta in "The Magnificent Ferengi". Ira Behr lamented casting one of his idols (and a legendarily rowdy rocker) as such a monotone creature.
Played with in-Universe in the episode "Little Green Men", when Rom notices the similarities between historical activist Gabriel Bell and Capt. Benjamin Sisko. Fans, of course, know that the two are one and the same.
J.G. Hertzler, most well-known among Deep Space Nine fans for playing Martok, also played Sisko's commanding officer, the Vulcan captain of the U.S.S. Saratoga in the pilot episode "Emissary". He also later played changeling Laas in Season 7's "Chimera".
Which means he played Changelings twice, no? Several times in the guise of Martok, and once as Laas.
PRINCE HUMPERDINCK as Martus Mazur, Quark's one-episode commercial rival in the Season 2 episode "Rivals".
Now if only they could just have put Zek in that episode somehow. Surely they could have found a way.
Bill Mumy, best known as Lennier in rival show Babylon 5, or Will Robinson of Lost in Space, appears in "The Siege of AR-558". His one stipulation for crossing over was that he not have to wear any more prosthetics.
Mumy is the only B5 cast member to think that far ahead; both Andreas Katsulas (Tomalak on TNG) and Mary Kay Adams (Lady Grilka on DS9) were stuck with rubber foreheads.
Speaking of B5: Captain Kate Lockley guest-starred as a Cardassian engineer who butts heads with O'Brien. (She later tried bumping something else, but alas, he's happily married.)
Melora is Dr. Grace Holloway from the '90s Doctor WhoTV movie.
Two actors from Breaking Bad appear at the beginning and end of the series; Jonathan Banks (BB's Mike Ehrmantraut) appears as an exiled criminal in Season One's "Battle Lines", while Raymond Cruz (Tuco Salamanca) plays a PTSD-suffering soldier in Season Seven's "The Siege of AR-558".
Romance on the Set: Alexander Siddig (Bashir) and Nana Visitor (Kira) got together and had a son, with Visitor's pregnancy being written into the show. They married the next year and divorced in 2001. Which leads to rather hilarious in joke in the episode where Kira says "This is all your fault!" to Bashir during an argument, since (in universe) he was the one that did the fetal transplant from Keiko to Kira. Funnier still because Bajoran labor is usually quiet and easy.
Science Marches On: Near the end of DS9, Dr. Bashir asks Odo to donate part of his natural goo to help figure out how to grow new organs. Come the early 21st Century, and humans have nearly perfected the ability to grow organs in a lab - not, admittedly, organs that can change to become the one that is required by an impending surgical procedure (which is what Bashir was aiming for).
Bashir wanted it to help find a way to grow organs on the battlefield - it's on thing to grow a new organ for a lung transplant when someone has been ill for a couple of months, but it's hard to know how many lungs, hearts, etc to have on standby on a battlefield, nor do you often have the time to grow them as the patient could die before they are fully mature.
Wag the Director: If you study Deep Space Nine's behind-the-scenes' trivia, you realize a good part of the regular and recurring actors became very involved with their character. Especially people like Andrew Robinson and J.G. Hertzler, who each wrote a whole novel about their character, Garak and Martok; or Nana Visitor, who met with the writers whenever she felt a scene was wrong about Kira. Another big name actor, Rene Auberjonois, was also involved in this character's development (requesting a new uniform for Odo, etc.), though he was a little more subdued—even if he didn't particularly care for this or that plot development:
Ira Behr: Being the pro that he was, he sucked in whatever dismay he was feeling—though I still got some of those vibes. He didn't do a screaming, "I won't do it, I won't play it! No no no!"
Visitor flat-out vetoed a romance with Gul Dukat, which had been building on mutual attraction from around the time of "The Maquis." The showrunners acquiesced, but later got their revenge by revealing Kira's mother to be one of Dukat's old mistresses. D'oh.
Re the shocking twist finale ("What You Leave Behind") of Sisko dying in the Fire Caves and becoming a Prophet, Avery Brooks was firm: Hell no, especially given the implication of an African-American male ditching his wife and newborn child. The writers suddenly saw his point, and the ending was rewritten to make Sisko's departure more ambiguous.
Out of all the cast members, Michael Dorn probably had the most pull. He wasn't interested in reprising his old role, but the showrunners were so desperate to have Worf cross over that they bent over backwards to make him happy. No longer would Worf be the big guy who got tossed around cargo holds! Interestingly, he was not involved in the decision to pair Worf with Jadzia, though he obviously made no demur.
Siddig wasn't too happy about his character being retconned into an Augment. He interpreted it as Behr/Berman et al. attempting to squeeze him into a "Data" role without consulting him, and responded by deliberately reading long equations and statistics in a drowsy, non-eloquent manner. The Augment stuff quickly faded into into the background and became all but irrelevant.
The cast hoped there would be a movie spinoff at some point (the TNG films still being successful at the time of DS9 ending) but the failure of Nemesis put paid to that idea. Any hopes of resurrecting the project would seem over - the actors have all seriously aged out of the roles (at the time of writing, it's been fourteen years since the show went off the air). The 2006 auction at Christies where a huge number of props (including the "hero" model of the station itself), costumes and other items from the series were sold off made it even less likely and the 2009 reboot of the entire Star Trek franchise would seem to be the final nail in the coffin.
Melora (from the eponymous season 2 episode) was meant to be the permanent science officer, but was replaced with Dax and reused as a one-off guest character — thus cheating poor Bashir out of a romantic interest yet again.
Alexander Siddig originally tried out for the part of Sisko (!), but was turned down for being too young.
More understandably, Andrew Robinson tried out for the part of Odo.
Michelle Forbes, playing the part of Ensign Ro, was originally supposed to be Deep Space Nine's token Bajoran in the cast. Forbes didn't want to commit to a full television series, so the new character of Major Kira was created. Ro would end up serving as the station's chief of security in the relaunch novels, however.
Robert Hewitt Wolfe planned to end "Second Skin" on an Ambiguous Clone Ending, with Bashir unable to determine whether Kira is actually a Cardassian. This left open the possibility that Kira was unknowingly a Cardassian agent all along, and that the Obsidian Order had been telling the truth.
When the Dominion invaded Federation space, Vulcan was pitched as one of the occupied planets. Ron Moore commented that Vulcan "carried too much weight," so Betazed got thrown overboard instead. Ironically, J.J. Abrams would happily blow Vulcan to smithereens in the 2009 movie. In an even more ironic decision, he blew Vulcan to smithereens for precisely the same reason that Ron Moore didn't have it occupied- it showed how serious things were, and that things were not going to be the same again.
The writers considered a Kira/Dukat Foe Yay scenario (season 4's "Indiscretion", introducing Dukat's half-Bajoran love child Ziyal, is artifact to where things may have headed). However, Nana Visitor (Kira) flatly refused to do anything of the sort, pointing out that he was guilty of the brutal occupation and near-genocide of Kira's species. Instead, the storyline was given to Kira's mother Meru as Dukat's "comfort woman" during the occupation in "Wrongs Darker Than Death Or Night." Similar to Michelle Forbes inadvertently causing the creation of Kira, this storyline probably worked out better for Kira's character.
Bashir's secret agent holo-program was intended to be a series staple, like TNG's Dixon Hill. Unfortunately, the people at MGM took it for an unauthorized Bond parody. Wonder where they got that idea, with names like Honey Bear and Mona Luvsitt?
Ira Behr tried (and failed) to get a Vic Fontaine-like character onto the show for years. Frank Sinatra Jr. was approached first, but as he would be playing a Rat Packer jazz artist (and thus be misconstrued for his father), he wasn't interested in playing that type of character. Robert Goulet and Tom Jones also turned the part down.
While "Trials and Tribble-ations" is regarded as one of the best episodes of the series and a fitting 30th Anniversary for the franchise, there were numerous other ideas which were tossed around. One was the return of Enfant Terrible Charles Evans from the TOS episode "Charlie X", most likely seeking revenge in a manner similar to Barry Waddle. Another was having the Deep Space Nine crew visit the mobster planet from "A Piece of the Action", where they find that the inhabitants have gone on to emulate Starfleet as a commentary on Star Trek fandom.
Speaking of "Trials and Tribble-ations," Dennis McCarthy originally wanted to re-record Jerry Fielding's kitschy score for "The Trouble With Tribbles" to go with the new episode's other recreations, but Rick Berman overruled him and demanded he write an original score. (Although he claims it wasn't in the budget.)
The writers originally toyed with the idea of killing off Jadzia Dax in the Season 6 episode "Change of Heart" rather than in "Tears of the Prophets", the season finale. The idea was that Jadzia would manage to convince Worf to continue their mission without her and leave her behind, in which case she certainly would have died. Worf would then have had even more angst than he ended up with to work through in the final season, having lost his wife out of choosing his career over her. Terry Farrell (Jadzia) was on board with it, but they ended up not going through with the idea.
The decision to ship Kira to Cardassia had a domino effect on the rest of the seventh season. Garak went along as backup, and Ira Behr didn't want to split Kira and Odo up, so Odo went, too. Originally, Odo was going to spearhead the search for a cure for the Changeling plague, culminating in a reunion with his "father" Dr. Mora who was actually responsible for engineering the virus as a weapon. The writers knew they would never have time to do this storyline justice, so Bashir and O'Brien went looking for a cure instead.
Word of Gay: Andrew Robinson has commented in multiple interviews that he considered Garak "omnisexual," and also strongly implied it in the character book he wrote. Robert Hewitt Wolfe has stated that he wrote Garak to be attracted to Bashir, but Bashir never realized this.
Word of God: Ron Moore stated that although the background check in "Doctor Bashir, I Presume?" ended up revealing that Bashir had been genetically augmented, this didn't preclude him from being the new template for the Emergency Medical Hologram; and that he did actually end up being selected. Therefore fans assume that either the (unseen) EMH Mark III or Mark IV was based on Bashir.