Fully Functional: Any episode with Data & Lore or their "father" Dr. Noonien Soong. (Also see the episode "Brothers" which was Acting For Three (including one stint in enough make up to make a Klingon's actor cry.) And let's not forget "A Fistful of Datas", where Data plays...a fistful of ancient Western stock-characters.
Similar to how Sisko does with baseball, Picard enjoys using Shakespeare as a metaphor for the human condition. Of course, everyone knows about Stewart's background in Shakespearean theater; he quotes Hamlet in "Hide And Q," and participates in Data's production of Henry V.
TNG had a minor in-show example: In "Descent, Part II" the Enterprise is forced to hide within a star's corona by using an experimental shield. The lieutenant at Tactical doesn't think that the shield will work, but is proven wrong. The actor played a different character in a previous episode who tried to make it appear that the shield didn't work.
In "Sarek," Wesley gets ticked at Geordi and taunts him by saying, "At least I'm not spending the night with a good book, like some people!" Geordi seems to take this remark rather personally.
In "Half A Life," David Ogden Stiers guest-stars as an alien scientist doing research work on the Enterprise. One of his report readouts is attempt number 4077.
Author Appeal: Baseball was Michael Piller's favorite sport. Shocking, we know. One of his goals was to bring that sport back to the 24th century, which had replaced it with Parrises Squares, Racket Ball and the like.; hence Dr. Stubs ('Evolution'). This also inspired the DS9 episode "Take Me Out to the Holosuite."
Behr: Baseball is Michael Piller's favorite sport, but in the first episode he ever wrote for Star Trek, he killed baseball. Why, we still don't know, but we thought we owed it to him to bring baseball back, even though he had chosen to kill it.
Banned in China: "The High Ground" got banned from broadcast in both the UK and the Republic of Ireland due to a line about Ireland being reunited in 2024 following a successful "terrorist" campaign.
"Conspiracy" was banned for a time in the U.K. because of the infamous "exploding Remmick" scene.
California Doubling: Lore's Rogue Borg compound in "The Descent" is The House of the Book performance hall and library building at the American Jewish University, Brandeis-Bardin Campus in Simi Valley, California.
Many episodes feature Riker playing the trombone, because Jonathan Frakes really does play trombone. And the episode "Data's Day" features Dr. Crusher teaching Data how to dance, because Gates McFadden is an accomplished dancer and choreographer.
Patrick Stewart reciting Shakespeare. Well, they had to get it in there somehow.
Both Patrick Stewart and Brent Spiner get a chance to show off their pipes. Picard leads his men in a sea shanty on not one, but two occasions: as an alien facsimile in "Allegiance", and in Insurrection (where he and Data sing "A British Tar" with relish).
Everyone in the cast sings, pretty well too. Brent Spiner cut an album of Jazz standards (and some new material) a few years back where his backup singers were Patrick Stewart, Michael Dorn, LeVar Burton and Jonathan Frakes, It was spectacular.
Averted in the episode Qpid, where the main characters engage in some medieval fighting in a fantasy recreation of Robin Hood. Only the men were given swordfighting scenes, in spite of the fact that the two female leads, Marina Sirtis and Gates McFadden, were the only ones in the cast who actually knew how to fence.
"Patrick Stewart WILL NEVER BE MENTIONED AGAIN in connection with Star Trek!" — actual memo from Gene Roddenberry. Fortunately, Patrick had a champion in Rick Berman, director Corey Allan, and others who helped rig the auditions in his favor. From the Blu-Ray TNG reunion:
Michael Dorn: Two seasons later, he's like, "...what is he still doing here?!" Jonathan Frakes: "I told them NO!"
With the high profile of The Next Generation securing his position, Roddenberry was able to leverage more control over the tie-ins set inside his universe. His archivist, Richard Arnold, immediately began cleaning house. The on-going DC Comics Star Trek series was immediately cancelled and reconned, replaced with a follow-up that would be much more in tune with the TV series. (Plus one that didn’t deal in as many original or “non-canon” characters.) Arnold also went to work on the novels, bringing his strict editorial sensibilities to bear. Arnold has had a number of choice comments about how he views the writers of tie-in materials, going to far as to suggest that many of the tie-in writers “had never written Star Trek”.
Berman was installed on the show by the studio as a way to keep a handle on the show: keep it under-budget, make sure that the scripts were done on time, filter out any adult material. Ultimately, Berman ended up in control because he played the politics game more effectively and Gene was completely toothless at this time, having driven away all of his allies and handed creative control over to his lawyer, Leonard Maizlish. Maizlish is the big bogeyman of the Trek world. However, he is credited with keeping the name "Data", casting John de Lancie and hiring Maurice Hurley, which resulted in the debut of the Borg.
One of Moore’s complaints about TNG was that the characters were not allowed to breathe as individuals or embrace other cultures besides Starfleet. You can almost smell his disdain in episodes he penned himself.
According to his book, Resistance is Futile: Assimilating Star Trek, Ira Behr had similar frustrations. Even after Gene passed away, his in-house directorial style lingered and made it difficult to affect lasting change on any of these characters. Both Ira and Moore would jump ship to DS9. That show was scripted in large part as an adolescent reaction to TNG and Gene's somewhat cranky views.
Descended Creator: Roddenberry's wife Majel Barrett, who also played Lwaxana Troi, owned a great deal of the rights and was one of the chief executives owning Star Trek after her husband's death.
Development Gag: "The Schizoid Man" was originally to have guest-starred Patrick McGoohan; the title of the episode is the same as that of an episode from his famous series The Prisoner. Even though McGoohan did not appear in the episode, the title remained unchanged as a tribute.
Directed by Cast Member: Stewart, Frakes, Burton and McFadden all directed episodes. McFadden also choreographed the dance sequences in "Data's Day", Frakes directed two of the movies, and Dorn directed several episodes of DS9.
Is what got Patrick Stewart an audition in the first place! The casting director saw him in a play and wanted him to try out, but Roddenberry was against it - feeling Stewart was too old and too bald to play Picard. Rick Berman admitted to also having doubts, but he was at least willing to give Stewart a shot. Berman, the casting director and production manager Robert H. Justman kept pressuring Roddenberry until he finally relented to seeing Stewart audition.
A minor example having to do with the score. "The Drumhead" is the last episode to be scored by Ron Jones before the brilliant Rick Berman (not the soundest policy maker in the Trek franchise) fired him to ‘try out new composers.’ There’s a reason why people vividly remember the score to "The Best of Both Worlds" but why nobody remembers anything from the last three seasons of TNG. DS9 was affected by this decision with the first three years worth of music proving largely unremarkable before they started using dynamic scores from David Bell et al. from season four onwards.
Executive Veto: Apparently Tracey Tormé has originally wanted to include an Andorian in the episode "Conspiracy", but was informed by a producer (probably Berman) that, “We don’t do antennae on this show.” As such, the Bolians were rolled in as a compromise: blue, antennae-less aliens. Later, DS9 writer and producer Robert Hewitt Wolfe reported that producer Rick Berman did not care for the antennae, and noted that, “if we’d been allowed to, I guarantee we’d’ve put an Andorian on the show so fast your head would’ve spun.“ The Andorians were finally given their day on Enterprise, appearing in the "The Andorian Incident", one of the least-regarded episodes in Star Trek history. (Looks like Berman had a point...)
Exiled from Continuity: Gene Roddenberry originally ruled that none of the TOS races and worlds (Vulcans, Klingons, Romulans...) would appear in TNG. The original characters as well as their possible offspring were also forbidden. This rule was obviously relaxed from the start, with the presence of Worf and Bones McCoy in the pilot.
Fake Brit: Daniel Davis, who played a hologram of Professor James Moriarty and the snide British butler on The Nanny, hails from the Royal House of Arkansas.
Picard's habit of straightening his uniform is called "the Picard Maneuver" by fans (not to be confused with "the Picard Maneuver" from the episode "The Battle").
Riker's... Unconventional method of sitting down (where he appears to mount the chair like a horse, caused by an old back injury and Johnathan Frakes' excessive height) is similarly known as the "Riker Maneuver."
The Galaxy-class starship is often abbreviated to "GCS".
Gender Flip: Wesley Crusher was originally a girl named Lesley.
Germans Love David Hasselhoff: Marina Sirtis has a sizable fanbase in Germany (She gloats in one interview that it's the only place where Brent Spiner doesn't have top billing).
Harpo Does Something Funny: Joe Piscopo was reportedly solely responsible for his character's dialogue and jokes in "The Outrageous Okona."
Hide Your Pregnancy: Gates McFadden was pregnant throughout season 4, including during "Remember Me" which contained a couple of physically demanding stunts that she performed herself (she wasn't aware she was pregnant at that point). Later in the season she is shown almost exclusively wearing her "lab coat" and/or being filmed from the sternum up.
In the William Shatner documentary The Captains, Patrick Stewart's behind-the-scenes experiences at TNG share an odd similarity with his character. Stewart, who was stressed out over succeeding the stars of TOS and his long hours, got the cast together and told them to quit goofing off between takes ("We're not here to HAVE FUN!!"); his castmates never let him live that down. In the long run, TNG taught him to do good work and have fun doing it. This is mirrored by Picard's final line of the show when he joins the Bridge Officers' poker game.
The Merch: Next Gen had two distinct toylines. The first, by Galoob, came out during the first few seasons. It featured 3.75" figures (in scale with Star Wars and smallish, die-cast vehicles including an Enterprise whose saucer detached. A few years later, Playmates Toys came out with its own line, which...Let's just say that Next Gen was a landmark moment in toys, even more so than Star Wars. The figures boasted superior sculpting and articulation (though sitting down looked slightly unnatural), there were aliens and villains right in the first run, and the line made incredibly creative use of electronics, with sound effects taken directly from the show! What's more, the show itself provided more than ample inspiration for variants, including the "Holodeck Adventures" line. Playmates held onto the license with its teeth, providing lines for Deep Space Nine, Voyager, the Original Series (which got its own badass boxed-set), the movies, Enterprise, and the first J.J. Abrams movie.
The Other Darrin: Spot. Yep. Data's first feline friend was a long-haired Somali, while later ones are played by a generic short-haired tabby. This has caused some Trek authorities to joke that Spot is a shapeshifter in disguise, or else lost his fur in a transporter accident.
It's old news now, but Dwight Schulz's transition from the half-deranged (and so half-sane!) "Howlin' Mad" Murdoch to the buttoned-down, mousey Reginald "Reg" Barclay was a novelty indeed. It paid off: Barclay is equally as — if not more — famous as his star-making role on The A-Team.
Brent Spiner was primarily a comedic actor before being cast as Data. Of all the TNG regulars, Spiner probably goofed off the most between takes, which is why Data is always wearing a semi-menacing grin in behind-the-scenes footage. However, he got to cut loose in "The Outrageous Okona", which had Data practicing his Henny Youngman routine in a comedy club.
TNG had a habit of casting noted TV "bad guys" in benign or heroic roles. Jonathan Frakes is the biggest example, as his career up that point had been almost entirely villain roles. Others include Harry Groener as Tam Elbrun, John Vickery as Andrus Hagan (he'll be back as a backstabbing Gul in DS9's "Final Chapter"), Ronnie Cox as Captain Jellico, Christopher McDonald as Richard Castillo, Paul Sorvino as Worf's brother Nikolai, Robert Knepper as Wyatt Miller (he would later play the heavy in VOY's "Dragon's Teeth"), and Spencer Garrett as Simon Tarses (he would go on to play one of the killer holograms in VOY's "Flesh and Blood"). No such luck for Marc Alaimo, though; he played four bad guys!
Promoted Fangirl: Whoopi Goldberg was a huge fan of the original series and has specifically named Nichelle Nichols as her inspiration for acting. Just before the second season entered production, she called Roddenberry, saying "I am a Star Trek fan, I was a Star Trek fan long before I was ever Whoopi Goldberg and I'm wondering if there's some part I can play in your show?" Roddenberry was so impressed that he re-wrote the bartender character he had intended to introduce for Goldberg.
The characters received new two-piece uniforms starting in Season 3 because the original one-piece suits were intentionally made one size too small (to look good on camera) and were causing serious back problems.
"The Defector" was supposed to open with another Sherlock Holmes pastiche, but legal issues forced the writers to retool it into a holodeck simulation of Henry V. This doubles as foreshadowing: Jarok, like King Henry, is forced to go undercover as a 'commoner' in this episode.
"The Best of Both Worlds" introduces a job opening for Riker on another ship, as well as a new female commander for him to butt heads with. The showrunners were grooming Riker to take over as Captain if Patrick Stewart didn't want to return.
Reality Subtext: LeVar Burton would often get headaches from the devices on his temples that were used to hold the VISOR in place. Likewise, Geordi would ocassionally mention headaches as a side effect of wearing the VISOR.
Probably an aversion with Worf; the DC Star Trek comics had long featured a Klingon in Starfleet named Konom, but his backstory and character are very different from Worf's and Roddenberry probably had a similar idea independently.
Data is derived from the character Xon from the cancelled Star Trek: Phase 2. Xon was a full-blooded Vulcan, the idea being that he would provide a contrast to the vanished Spock—whereas Spock was always denying his human side and embracing his Vulcan, Xon would already be secure in his Vulcan-ness and thus be more intrigued and curious about understanding human emotions. This eventually mutated into the idea of an emotionless android seeking to achieve those human emotions.
Tony Todd as Worf's brother, Klingon Captain Kurn.
Okana's squeeze, Transporter Chief B.G. Robinson, is played by Teri Hatcher. Ironically this was an uncredited role.
Ashley Judd as Ensign Robin Lefler. It's like the show is poking the eye of everybody who damned Wesley as a geeky nuisance with no balls.
Famke Janssen as Kamala, the empathic metamorph who bonds with Picard in "The Perfect Mate." Famke Janssen and Patrick Stewart would later star together in the X-Men film series as Jean Grey and Professor X, respectively.
Romance on the Set: Patrick Stewart, on the lookout for the next Ex-Ms. Patrick Stewart, dated Jennifer Hetrick (Vash) during Seasons 3 & 4. They were briefly engaged during "Q-Pid", but nothing came of it. In 2000, six years after TNG ended, Stewart and producer Wendy Neuss did marry. They divorced three years later.
"Skin of Evil" was Denise Crosby's last aired episodenote other than her coming Back for the Dead in an Alternate Timeline in "Yesterday's Enterprise", with her character being killed off. In the previous episode, "Symbiosis" (which was actually filmed later), she's in the background at the end, as Picard and Crusher enter the turbolift. Just as the doors close she waves goodbye to the camera.
In "QPid", Vash has been transformed into Lady Marian by Q. As they were filming the scene where she paces back and forth in her cell, Jennifer Hetrick tripped over her dress. The director left it in reasoning that a 24th century woman would not be used to walking in 12th-century finery.
Troubled Production: As per usual with Trek, getting the show on the air was extremely taxing. Gene was quarrelsome, in ill health, and fans of The Original Series were more than usually scrutinizing. The prototype uniforms smelled bad (spandex retains bodily oils more than the newer cloth uniforms) and gave the actors back problems. By the time three seasons had passed, three actors (Crosby, McFadden, and Muldaur) had had enough, and Patrick Stewart was in the middle of contract re-negotiations. General consensus has it that the audience response to "Best of Both Worlds" revitalized the program, although the addition of Whoopi Goldberg and return of Gates certainty helped.
The doomed science vessel in "The Naked Now", the SS Tsiolkovsky, has a plaque stating that it was built in the USSR.
Possible, despite real-world politics, given Trek's timeline with the Eugenics Wars and World War 3
In "The Royale", Picard refers to Fermat's Last Theorem as having been unsolved for over 800 years. Whoops. This was later corrected in the DS9 episode "Facets".
Also in "The Royale", debris from a NASA spacecraft, which according to this episode took part in a mission in the mid 21st century, are beamed aboard the Enterprise. However, the NASA "Worm logo"◊ seen on the debris was actually replaced by the "Meatball logo"◊ in 1992.
Wag the Director: Patrick Stewart wasn't too thrilled with his stodgy, preachy, apparently sexless Captain in Seasons 1-2, and intimated that he might leave the show if something wasn't done about it. (Actually, the phrase he used according to Ron D. Moore was "there isn't nearly enough shooting and screwing on this show" before he bid a cheerful goodbye.) British TV is flooded with this type of character, particularly in the 70's and 80's, and Stewart had had his fill of patriarchal dispensers of wisdom. This led to "Captain's Holiday", which was tailored-made to please Patrick.
Robin Williams was offered the part of the time-traveling "Dr." Berlinghoff Rasmussen in "A Matter of Time", but he was busy working on Hook at the time, so the role was given to Matt Frewer.
Stewart auditioned for TNG three times. Once for Data and twice for Picard. Once bald and once wearing a hairpiece.
Edward James Olmos was offered the part of Jean-Luc Picard, but declined due to being constrained by contractual obligation. Over time this story has morphed into Olmos being considered for the lead in Voyager, and other such rumours.
Marina Sirtis and Denise Crosby were originally going to play Yar and Troi respectively, but switched roles at the last minute.
The Borg were planned to be insectoid aliens but the special effects budget wouldn't cut it. The concept was reused for the Jarada, who we never see on-screen.
The writers considered crashing the ship's saucer section on a planet as the cliffhanger ending of Season 6, but the budget didn't allow for it. This was eventually used in Star Trek: Generations instead. Separating the saucer itself was planned to be a much more common tactic, but again effects expenses meant it was only used four times. In addition, halfway through season one it had become painfully obvious that saucer separation was time-consuming and destroyed the pacing, a major issue when it was supposed to happen halfway through dramatic battle sequences.
Wesley Snipes was considered to star as Geordi LaForge. Tim "Tuvok" Russ also auditioned for the role. He came that close to getting it, too.
Among the actors that auditioned for Riker were Jeffrey Combs and Vaughn Armstrong - both of whom became pretty well-known to viewers regardless.
How much better would "The Outcast" have been if the actors playing the sexless J'naii were male instead of female? Jonathan Frakes criticized the staff for not doing that and making the message of the episode that much stronger.
For "All Good Things...", the writers considered revisiting "The Best of Both Worlds" in addition to the other three time periods.
Dr. Selar (from "The Schzoid Man" and repeatedly referenced) was originally intended to be a recurring character and would develop a romance with Worf, since there was a drought of Vulcans on the show and pairing one up with a Klingon sounded interesting. But this was scuttled when K'Ehleyr was introduced later the same season. Incidentally, both characters have similar names (seriously, try sounding them out) and were played by Statuesque Stunner Suzie Plakson.
The Enterprise was originally going to be the seventh rather than fifth ship to bear the name, and was referred to in the script as the Enterprise Seven rather than the Enterprise-D we know and love.
The writers considered bringing back Denise Crosby as Tasha in one of the alternate universes in "Parallels", but decided against it, feeling it'd be redundant after "Yesterday's Enterprise".
The Q Continuum were originally going to be many people with the same face. This portrayal is rather apparent in "Encounter At Farpoint," with each change of costume Q goes through usually resulting in a change in attitude and demeanor. However, by Q's next appearance, this particular idea seems to have been dropped, with Q acting as a single individual, and later episodes (and Star Trek: Voyager) introduced additional members of the Q Continuum.
The writers didn't know what to do with Troi in Season 1, even omitting her from four episodes just to save themselves the trouble. Marina Sirtis has said she was nearly let go after Season 1 because of this. She was only kept on because Denise Crosby had already left and Gates McFadden would soon decide to leave.
At one point Wesley Crusher was planned as a female character named Leslie Crusher, as the TNG creators thought the teenaged boy genius was too much of a cliché. (They were probably right.) Although considering that a female genius would have been Mary Sue, it was kind of a no-win situation.
Ronald D. Moore considered bringing Captain Jellico back to be the commander of the Enterprise in the Alternate Timeline presented in the episode "Tapestry".
One of the people who was considered a favorite for the role that eventually became Tasha Yar was Rosalind Chao, who later portrayed Keiko O'Brien.
Patrick Stewart's contract was up for renewal at the end of season three, and he waffled a bit over whether he would continue with the show. Hence, "The Best of Both Worlds Part 1" was constructed around the possibility of losing Picard (with no forward planning on how to resolve the story either way).
Word of Saint Paul: The Star Trek The Next Generation Technical Manual, in its entry on the Captain's Yacht, includes an out-of-universe sidebar that says "Patrick Stewart informs us the yacht is named Calypso after Jacques Cousteau's vessel", in a way that suggests that, as far as the writers of the Manual are concerned, he should know. This was never confirmed on screen, since the Enterprise-D yacht was never used (or even confirmed to exist), but was nodded to in Star Trek: Insurrection, where the yacht on the Enterprise-E is called the Cousteau.
Also from the technical manual, Rick Sternback had a pet idea for "Cetacean Ops" — literally, navigation research that's being handled by a dozen dolphins being overseen by a couple of whales — in huge underwater tanks throughout the ship. It got a mention in "The Perfect Mate", but goes otherwise unregarded by other writers.
‘Some day I’m going to be a Starship Captain!’ says Rene Picard, which is almost a portent of the future since David Tristan Birkin would go on to play Baby!Picard in "Rascals."
Hey, it's Zefram Cochran as Prime Minister Nayrok in "The Hunted" (and Zaglom Shrek in "Birthright," Pts. 1 & 2, even though you can't see his face in that one)!
Marc Alaimo appeared as a minor Rubber-Forehead Alien in Season 1's "Lonely Among Us," Romulan Commander Tebok later that season in "The Neutral Zone," Cardassian Gul Macet in Season 4's "The Wounded", and a 19th century gambler in Season 5's "Time's Arrow" before taking on the role of Deep Space Nine's Gul Dukat.
Hey, that Ferengi Dr. Farek in "Menage a Troi" sure sounds a lot like Neelix... At one point on Voyager ("False Profits"), Neelix is forced to get makeup and surgery so that he looks like the Grand Nagus of the Ferengi, and the actor pulls it off again with great aplomb. Said actor would go on to be one of the 'first' Ferengi ever, in the ENT episode "Acquisition".
W. Morgan Sheppard is one of those names that crops up in fantasy TV, appearing in everything from Trek and Babylon 5 to Doctor Who. He is first seen here in "The Schizoid Man" as Data's 'uncle'. You can also spot him as a Klingon in Star Trek VI and the Ahab-like alien in VOY's "Bliss". Most recently he played a Vulcan science minister who Spock told to get stuffed (figuratively) in the Abrams movie.
Roy Brocksmth had one-shot appearances here and in DS9: As a Zakdorn in "Peak Performance" and a Bajoran smuggler in "Indiscretion". His most famous role, however, is Dr. Edgemar in Total Recall.
James Sloyan is an authoritative actor who has played four aliens, usually with some sort of dark secret attached. He also died often: Admiral Jerok in "The Defector" (suicide), Future!Alexander in "Firstborn" (erased from history), and Jetrel in the eponymous VOY episode (terminal disease). He also had a recurring role on DS9 as Odo's "father", Dr. Mora.
Eric Pierpoint had guest roles in all four spinoffs: a shape-shifter who attempted to try it on with Picard ("Liaisons"), Cpn. Sanders in "For the Uniform" (DS9), the Klingon answer to Judas Iscariot in "Barge of the Dead" (VOY), a big game hunter in "Rogue Planet" (ENT), and a Section 31 honcho in the Terra Prime arc (also ENT). He is probably best known for his role as Det. Francisco in the cult show Alien Nation.
John Vickery had a good hit rate: Going from a (mute) Betazed in "Night Terrors", to a Cardassian Gul in four episodes of DS9, to Klingon prosecutor in an ENT episode, "Judgement". Babylon 5 aficionados know him as Neroon.
Suzie Plakson as Selar, K'ehlyr, and the female Q on Voyager to name one.
Carolyn Seymour plays a Romulan in "Contagion", an alien scientist who chooses to leave her people in "First Contact" and finally reprises her Romulan role as Captain Toreth in "Face of the Enemy".
Look out for the future Tuvok (Tim Russ) playing a human terrorist in "Starship Mine". (And, ironically, being the recipient of a Vulcan nerve pinch.) He also plays an unnamed human bridge crew member in the 23rd century in Generations.
Marc Alaimo who would become, in Deep Space Nine, Gul Dukat, played 4 different characters in TNG, including the first Romulan seen in TNG in "The Neutral Zone". Most notably he played the first ever Cardassian seen in Star Trek (Gul Macet in "The Wounded").
Robert Duncan McNeill, Voyager's Tom Paris, as Nicholas Locarno in "The First Duty." (The character of Locarno was the inspiration for Paris. The Voyager creators say they didn't plan to hire the same actor; once they realized they had, they considered making McNeill Locarno on Voyager, but reformulated him into Paris, feeling that Locarno "couldn't be redeemed enough" (read: they didn't want to pay royalties) for what they planned with Paris.
Ethan Phillips, Voyager's Neelix, as Dr. Farek in "Ménage à Troi" and the holographic maître d' in First Contact.
Most jarring of all is James Cromwell as the leader of a potential new Federation alliance world in "The Hunted", when he later played Cochrane in Star Trek: First Contact.
David Tristan Birke, who played Rene, Picard's nephew in "Family", later played the young Picard himself in "Rascals".
Max Grodenchik as the very typical conniving, treacherous Ferengi Sovak in "Captain's Holiday"; better known for his later role as the very atypical (and somewhat dim) Rom from Deep Space Nine.
Armin Shimerman played both Letek, one of the first Ferengi ever shown onscreen in "The Last Outpost", another Ferengi, Bractor in "Peak Performances", and the better known Quark — also from Deep Space Nine.
Majel Barrett (who played Nurse Chapel in Star Trek TOS, as well as Number Two—actually called "Number One"—in the original pilot) as Lwaxana Troi, and also the voice of the ship's computer in both series.
Diana Muldaur, who played Dr. Pulaski in season 2, had two previous spots on the original series (as different characters, both doctors, no less.)
Christopher Collins, AKA Chris Latta played a Klingon Captain in A Matter of Honor and later plays a Pakled in The Samaritan Snare. Might be more of a case of You Sound Familiar.
Michelle Forbes played a small role in season 5's "Half A Life" before coming back in Season 6 as semi regular Ensign Ro Laren.
Charles Cooper played a drunken and disgraced Klingon general in Star Trek V: The Final Frontier. Here, he got to play the much more dignified Chancellor K'mpec in "Sins of the Father" and "Reunion."
Urban Legend of Zelda: The operator's manual for the Star Trek: The Next Generation pinball mentions a "secret Video Mode" in the gamenote allegedly a version of Breakout, and there are two operator settings to control it. Problem is, no one knows how to activate it, and it's unclear if the mode is actually enabled in the final game or not. It can sometimes be seen on the display in the game's attract mode, however.