In the later seasons of Boy Meets World, characters constantly reference how Corey and Topanga have been in love since preschool. This is odd to anyone who rewatches the first season in which Corey heavily antagonizes Topanga. This is retconned at one point when Corey claims he went through a phase of thinking "girls are icky". However this retcon is supported though, in a few season. One episode, most notably is "Boy meets Girl" (the second to last of the season) where Amy thinks its odd that Corey is going on a date with Topanga since he often antagonizes her. At that thought she then think it actually makes perfect sense. The rest of the episode is a good starting point for the Corey/ Topanga relationship with Corey just maybe having feelings for Topanga. Other episodes that hint at something between them are "Model Family" and "Corey's Alternative Friends"(her very first appearance.) These episodes support the notion that Corey was possibly just ending his girl's are icky phase possibly antagonizing Topnaga because he liked her as Amy suspects as pointed out above.
Lampshaded when Corey begs Topanga to hearken back to her Cloud Cuckoo Lander habit of believing everything will always work out and be all right when his baby brother's life is in the balance. He eventually succeeds when she ends the episode drawing hearts on both their faces in lipstick and thanking him for reminding her it was okay to be idealistic.
In a season 1 episode, the guys' mothers all descend on to the house to make sure they're running the house right, and thus the boys have to clean every nook and cranny in that house. Should be no problem for Danny Tanner, a man so obsessively clean he cleans his cleaning products, but in this episode he's not very happy at all about having to scrub up the place.
Jesse also suffered from a milder form of this. In early episodes, once or twice, he was shown enjoying sports on TV or playing something simple with the others (one episode had him betting on a basketball game with Joey). This is the same Jesse who would later have an episode dedicated to his inability to play basketball. Also, he wasn't always Greek and they changed his last name at John Stamos's request to better reflect the actor's own Greek heritage.
MacGyver used a gun in the pilot episode of his series. Another episode in the first season ("Countdown") says he served in Vietnam, which seems incredibly unlikely given the rest of the series. They acknowledge that in a way. When they pair Mac up with the Phoenix Foundation in the second season, they also did an episode rewriting his meeting with Pete. In the first season, they're depicted as having met for the first time ever in a combat situation where upon they exchanged info and Pete recruited Mac to join his govt agency. In the second season, they meet for the first time ever in a US City. While running a cab for his buddy, Mac ferries an enemy agent. He notices them being followed, and accidentally foils Pete's sting. The creators state that this is because they wanted to quietly retcon his military service.
Dr. Zachary Smith of Lost in Space was originally going to just be the villain for the first few episodes and then get killed off. He was written as an evil, murderous man who would even kill children to get what he wanted. But the way Jonathan Harris portrayed him was so entertaining that he was spared and became part of the main cast as the JerkassLarge HamToken Evil Teammate everyone knows and loves.
Angel, Buffy and Darla in had remarkably different personalities in the first few episodes; Angel was mysterious and kind of chipper (especially his first appearance), Buffy was a perky cheerleader and Darla was whiny, cowardly, not particularly smart or capable, and seemingly not even all that important in the Master's hierarchy (Luke, for one, clearly outranked her). It wasn't until the episode "Angel" that they settled into the personalities they are better known for; Angel became brooding, Buffy was a kind of grim optimist and Darla had a distant, haunting persona (since she died in this episode, this is better seen when she returns from the dead in the show Angel).
Anya was also initially manipulative and infiltrated herself into Cordelia's social circle almost immediately. Later she developed a No Social Skills personality that left her unable to understand much of human interaction and claimed she had a Villains Never Lie attitude when she was a demon.
Harmony was an obnoxious but not particularly stupid Smug Snake in her earlier appearances before turning into the airheaded Harmless Villain she is known for. Being turned into a blood sucking demon just might have something to do with that: Other vampires lose their soul. Harmony lost her brain.
Something like this happened to Buffy's dad once he stopped appearing. In the first couple of seasons, he seemed like a decent guy whenever he visited. Starting with a broken promise in "Hopeless", however, he morphed into a quintessential deadbeat dad who couldn't even show up for Joyce's funeral. It says something that his last two appearances are Buffy's memory of a Cosmic Retcon and the Cuckoo Nest episode.
Clem was kind of a jerk in his first appearance. On the other hand, it could just be that kittens are his preferred meal.
On Angel, Lilah Morgan doesn't develop into the uber-bitch role she's known for until season 2. The few season 1 episodes she's in have her personality remarkably different. Take "Five By Five" where she gets threatened by Faith. Season 2 Lilah certainly wouldn't have been intimidated.
In the first season of Doctor Who, the First Doctor is an acerbic, prickly personality who gradually grows into a lovable eccentric over the later seasons. In a frequently-cited example, in the first Doctor Who story, "100,000 BC", the Doctor was about to "mercy kill" a caveman with a rock so that he couldn't slow the rest of the party down. His companion Ian Chesterton stops him, the Doctor mutters an excuse and they go on. In the next story, "The Daleks", in another commonly mentioned example, he deliberately sabotages the TARDIS so that they can stay on Skaro and explore. By Season 1's "The Reign of Terror", he starts charging around meddling with things and getting in way over his head with hilarious consequences, and by Season 2 he declares himself a defender of Earth while talking down a Dalek, a big difference from his True Neutral characterisation before. From that episode onwards he's an enthusiastic, benevolent, sharp-tongued Creepy Good eccentric with a distinctively sneaky and ruthless streak and a real love of getting himself into trouble, then back out of it - all of which remains constant for him and for all other Doctors.
This is lampshaded in "The Chase", in the episode "The Death of Doctor Who" - Barbara correctly identifies the Doctor's evil Dalek robot duplicate in a Spot the Impostor situation because the robot tells Ian to bash the double over the head with a rock, making Barbara exclaim that the Doctor would never do that (even though he tried to in "100,000 BC"). It can be speculated that the 'death' in the title is actually the 'death' of the Doctor's old characterisation.
The Second Doctor was the first time they regenerated the character and no-one quite knew what effect this could, or should, have on the Doctor's personality - only that he would have to be different to the First Doctor. He started out being written as an interstellar 19th-century ship captain, then as a 'Mr Hyde' version of the First Doctor who took on distorted, dark versions of his quirks, before settling down into the cuddly but dangerous Hobo character he'd be for the rest of his run. Fortunately, this worked really well - this initial confusion over his personality was explained eventually as the result of the shock of undergoing an obviously physically traumatic process, and became a stock feature of regeneration ever after. Later Doctors, even ones whose personalities have already been planned out in full, traditionally kick off with a story in which they are completely loopy (ranging from confusion to yelling non-sequiturs to thinking they're their previous incarnations to trying to murder their companions) before settling into their main personality.
This was cleverly used in the Fourth Doctor's first story, "Robot" - thanks to production scheduling, Pertwee's producer Barry Letts was forced to produce it instead of the producer lined up for the new Doctor, Philip Hinchcliffe. Having the difficult task of establishing the new Doctor as different from Pertwee while having no idea what Hinchcliffe planned on doing with the character, Dicks wrote the Doctor as a broad clownish comedy character (with some inspiration from Harpo Marx) but establishes that he's in a loopy, unsettled post-regenerative state for almost the entire story, only indicated as settling down into his real personality at the very end of the episode where the Doctor injures his hand on a brick now that his overdriven physical processes have worn off. While a lot of the Fourth Doctor's quirks are established in this episode (such as the natural funniness, the childishness, the bottomless pockets, his fondness for jelly babies and his resentment of all authority) Hinchcliffe took the character into a more Gothic Horror influenced direction, giving the Doctor a brooding and Byronic side to add an edge to his funniness, and playing his capriciousness and unpredictability For Drama at times as well as for cheap laughs. Even when he became a comedy character again later, he became a playful wit rather than a clown.
Some companions suffer from this as a result of writing artefacts:
Steven was introduced in his first full story "The Time Meddler" as a Loveable Rogue and extremely hot-tempered character who (being a space pilot) was already familiar with the way most things worked, condescendingly calling the Doctor 'Doc' and knowing lots about aliens and time travel science. In "Galaxy 4" he suddenly develops a much less hot-headed way of voicing disagreements, due to the serial being written for Barbara, and his lines just being hers with some tweaks. In his third story he takes a middle route between these two characterisations - the more sombre and respectful air of "Galaxy 4" while getting his confrontational personality from "The Time Meddler" back, which remains his default personality from then on.
Jamie is a fairly severe example, as he was not originally intended to be a companion and was added on after they'd already filmed a scene of the crew saying goodbye to him, with the next two scripts ("The Underwater Menace" and "The Moonbase") written without his character. As a result, in "The Underwater Menace", while he's a fairly active player in events, his dialogue and actions are mostly pilfered from Ben, Polly and some of the Ragtag Bunch of Misfits characters in the episode and rewritten in Scottish dialect, leading to a lot of out of character behaviour (notably, slapping Polly).
In early episodes of It's Always Sunny in Philadelphia, Mac actually displayed some fairly liberal, progressive viewpoints, and chewed out Charlie for not having any minority friends. This would seem quite jarring to viewers familiar with the later seasons, since he's established as being quite homophobic and transphobic.
Star Trek: In early episodes, Spock wasn't quite yet the emotionless Vulcan we all know him as and was even seen to smile a few times. In a scene from "The Cage" where the aliens snatch two female crew members, Spock shouts, "The women!" in a very emotional manner. The end of "The Enemy Within," where after Kirk's Evil Twin attempts to rape Yeoman Rand, Spock leeringly teases Rand about the duplicate's "interesting qualities," surely the most misogynistic moment in the entire Trek canon. Spock hadn't been given his emotionless personality because that was meant to be part of Number One's character. The network was not comfortable with the idea of a cold, unemotional woman (let alone one with a measure of command authority), so the character was scrapped and the trait transferred to Spock. Leonard Nimoy has admitted that in early episodes he was mainly playing Spock as a military officer. In "The Corbomite Maneuver", there's a scene where the Enterprise is seized by a gigantic and apparently hostile vessel and Spock merely says "fascinating" (for the first time). Nimoy has cited this as the moment the character really "clicked" for him, although it still took a few more episodes for Spock to fully settle into his stoic characterization.
Data went through a similar period of uncertainty during the first (and to an extent, second) season of TNG. It's not clear at first whether he's supposed to have emotions or not, as he grins awkwardly, gets drunk and subsequently seems to act on sexual desire, and often speaks in an oddly musical tone. He also uses contractions. The show only cemented its characters and premise in the third season, and that's when Data's famously emotionless and inquisitive personality really took hold. Fortunately, a conversation he has with Riker in the first episode does help pave the way for his future characterization.
Picard briefly carried on Chekov's habit of attributing everything to his home country. This running gag ended quickly, and seems very strange in light of the extensive knowledge of history and culture that he displays later on.
In the TNG pilot "Encounter at Farpoint," Picard implies that the Ferengi (who had yet to appear onscreen) were known for eating their nominal allies, a trait that seems unlikely now. Early appearances show them as a fairly warlike race, meant to replace the Klingons as primary antagonists, but ended up as the greedy, unscrupulous capitalists shown in most of their appearances. This is at least justified in being rumors being that the Federation hadn't actually established contact with the Ferengi yet.
In an early episode, it's stated that Riker wants to be a ship's captain more than anything else.
Riker is a good example of this overall, as lampshaded by Q:
Riker: I don't need your fantasy women.
Q: Oh, you're so stolid. You weren't like that before the beard.
In an early episode, Odo realizes that Quark was lying to him because Quark told him Rom fixed the replicator, which he deduced was impossible because Rom is an idiot. But Rom is later established as a technical genius, who thoroughly impresses Chief O'Brien with his ability and efficiency. It would be considered just a character mistake, except Odo is firmly established as being very good at observation and "sizing people up." This is eventually lampshaded by Rom's assertion, "I've always been smart, brother. I just lacked self-confidence."
Rom's first appearances had him acting much more like a regular Ferengi than his later My Species Doth Protest Too Much persona. In one first season episode, Rom gets tired of working for Quark and attempts to kill him by flushing him out an airlock. Even in season two, when Quark gets shot, his mood improves considerably when he's reminded he'll inherit the bar if Quark dies and he's horrified when he inadvertently saves Quark's life. All of this seems very at odds with the meek and gentle Rom of later seasons.
The Klingons weren't always the Proud Warrior Race they were in later series. On TOS, they could be quite deceitful, sometimes conducting espionage within the Federation ("The Trouble with Tribbles"). One of their number, who's portrayed as very much a Dirty Coward, even manipulates a Proud Warrior Race, who eventually turn on him after deciding he's "without honour" ("Friday's Child"). You know, like the later era Klingons would do. It was the Romulans who were the honorable warriors Starfleet faced ("Balance of Terror", "The Enterprise Incident"). But starting with TNG, these characteristics were firmly flipped between races, as Klingons became the race of proud warriors while the Romulans were established as a race of Smug Snakes.
Margaret Houlihan went from hypocritically strict Battleaxe Nurse to feminist heroine. It's pushed aside with some Character Development, but the show still tried to Retcon her into being a decent person all along and that everyone just didn't understand her enough.
This was a prime example of Writer on Board, as it reflected the makeup of the show's writing staff at the time of the portrayals. Originally, when the show began, the writers were all male, and were mainly interested in staying as true to the original book and film as possible. However, once Linda Bloodworth-Thomason was hired as the show's first female writer, she immediately began downplaying or even eliminating almost every single negative attribute of Margaret's. It's not really surprising that Ms. Bloodworth-Thomason would do this, given the fact that she later became famous on her own account as creator and executive producer of one of the most ultra-feminist programs on American television, Designing Women.
John Cage was introduced in Ally McBeal's second episode as the slightly odd founding partner of Cage, Fish, & Associates who frequents prostitutes for the purposes of sex without romance (and is set to go before a judge for his latest dalliance, represented by a disgusted McBeal). A year and a half later this is mentioned in front of the by-then fleshed out quirky, mercurial, and lovable Cage (now McBeal best friend) and his stunned girlfriend; Cage's defense for this is that he hadn't "found his character yet".
Sabrina was much more of a Shrinking Violet and desperate to fit in, often needing reassurance from her confident best friend Jenny. In season 2 onwards Jenny was Brother Chucked and replaced with Valerie - who took on the role of awkward wannabe and left Sabrina to become more of a Cool Loser who isn't actually bothered by Alpha Bitch Libby's taunts.
A few episodes had Harvey being oblivious to Libby's Alpha Bitch tendencies, thinking she was nice because she was to him. Later on in the season he's fully aware of what a nasty piece of work she is. Additionally Harvey is a lot more clueless and Oblivious to Love in early episodes - before getting a bit smarter in season 2.
Aunt Zelda in season 1 was the Only Sane Man and a strict disciplinarian. While still the responsible aunt in later seasons, she got a lot more Not so Above It All moments and took part in a lot more slapstick. Her overall character became a little softer.
In Scrubs J.D.'s (and to a lesser extent Elliot's) personality quirks didn't really gel until the second season. A notable example is his switching from drinking beer to watered-down appletinis. (Although in the eighth season, he mentions that while he has had beer before, he simply prefers appletinis because they make him feel fancy.)
He also had sports memorabilia in his room. This is the guy who now thinks that basketballs come "three to a can".
This particular case of characterization is due to a Throw It In moment where Zach Braff mentioned to series creator Bill Lawrence that he knew almost nothing about sports. Lawrence, recognizing its comedic potential, made that part of J.D.'s personality.
In the early episode, Dr. Cox calls J.D. by his name a few times. After that, it's always "Newbie" or a girl's name. Although that has been broken on a few occasions. The 5th season episode "My Fallen Idol" had him use it in a touching moment while thanking him for his emotional support. Also, there are times when Cox has addressed J.D. by his surname "Dorian", but that's only when he's super pissed, or after J.D. leaves the hospital in season 8 and they become equals.
Another one with Dr. Cox that is big because of how prominent it is...the punchline of one episode is that he does, in fact, have friends, or at least a bunch of guys to watch the game with. When later episodes make a big deal about how he doesn't.
In the first several episodes the Janitor was a lot meaner, fairly humorless and never did anything except randomly clean and terrorize J.D. This is partially explained that the character wasn't meant to go beyond the first episode, and the entire first season was written as him possibly being just a projection of J.D.'s paranoia and self-loathing, he wasn't acknowledged by or interacted with any other character until the first episode of the second season. He became more Affably Evil and Cloud Cuckoo Lander as the show went on (and Neil Flynn started improvising more and more), with his personality fully gelling as a man who desperately wants respect in the world but doesn't care if people like him.
In the second season Elliot was made to be unable to use dirty words, often making up funny euphemisms such as "bajingo" for "vagina". However in a few season 1 episodes she uses the words "penis" and "vagina" normally.
In the first season, Elliot also was portrayed as a competitive and smug know-it-all. Her neediness and neurotic tendencies didn't properly surface until the second.
In the Gossip Girl pilot, Chuck was a psychotic who almost rapes Blair. Now, they have a sexual-tension filled relationship.
Actually, Chuck's not weird for just that reason in the pilot. There's also his rage issues and his lack of control, which is completely at odds with his careless, Manipulative Bastard personality of later episodes.
Phoebe's boss Elise in Charmed was introduced as the boss from hell and the rest of the season 4 episodes reflect this. However all her following appearances on the show have her more friendly with Phoebe and the rest of her co-workers.
An example brought on by Executive Meddling. Paige began season 5 as a lot more promiscuous - Rose McGowan eventually going to producers and saying "Paige isn't a ho!" - so this trait vanishes completely.
Paige and Henry are at each other's throats in the latter's introduction episodes. The Slap-Slap-Kiss element of their relationship gets dropped pretty soon. Additionally Henry seemed rather cynical in his debut but this trait didn't show up in other episodes.
Kirsten from Party of Five was the show's only real normal character as a nanny for Owen in season 1. In season 2 a bit of a wacky side emerged, particularly when confronted by her annoying mother. This gets subverted in season 3 when she ends up suffering from depression.
In the pilot of Stargate SG-1, Jack O'Neill is seen looking through a telescope on his roof and seeming fascinated by the heavens. By the end of the first season, he was already The Watson with no interest in astronomy or any other kind of science. And from there he would only become more clueless about anything that didn't involve Stuff Blowing Up or being a Big Damn Hero.
They made several callbacks to Jack's knowledge of astronomy and telescopes over the course of the series. In Singularity, he was the one who gave the technical description of a black hole's accretion disk, before remembering that he was pretending not to know stuff like that, and he stays behind on the planet to run the telescope observing the black hole. In 1969, he's the one who goes to the observatory, to use the telescope to confirm that the times the general gave them were for solar flares.
In the very early episodes, Samantha Carter was a borderline Straw Feminist. She was still a feminist in later seasons, but after the first few episodes she decided to shut up about it already and actually be helpful. Her early characterization is thus over-the-top and rather shallow compared to the character she was for most of the series. At several points they revisit the infamous "reproductive organs" speech from the first episode, and intentionally make it sound as awkward and silly as it really would be. In the director's cut of the premiere, they removed that line entirely.
Believe it or not, in the early episodes of Law & Order: Special Victims Unit, it was Olivia Benson who was the fly-off-the-handle-violent-templer hothead partner and Elliot Stabler who was the stable, analytical, don't-let-it-get-to-you partner.
Dr. George Huang's first appearance is vaguely sinister, with his perv-stache and line delivery suggesting his interest in the criminal mind is a little creepy. Less than a season later, he got a full makeover, wears dapper suits, and is a sensible, trustworthy ally whom both Benson and Cabot have gone to for personal advice.
In an early episode of How I Met Your Mother, Marshall tries to fight a guy who he thinks hit on Lily, and when he finds out he's gay, he's incredibly relieved, saying "I've never been in a fight before." Yet, by the fourth season, we found out that he and his brothers used to fight quite brutally and has an Offscreen Moment of Awesome by beating violence-crazy barman Doug.
In another early episode we hear Robin lament she never played team-sports, saying she instead played singles tennis in high-school. In a later episode we find out she was in fact a member of a hockey team in her teens.
A Season 9 episode has Robin saying that she doesn't have any female friends because she was raised as a boy and that she sucks at making them because she finds them annoying. She's never showed this much animosity toward females in the past and even has had female companions outside the group in previous seasons.
The characters' general personalities take a few episodes to really gel as well. Robin isn't nearly as quirky, brash, or hardnosed as she would become near the end of the season, Lily is a lot more gentle, Willow-y, and socially-conscious than she's known for, Marshall is kind of shy and quiet rather than being energetic and extroverted, and Barney's more of a sitcom-typical suave womanizer rather than the very specific form of over-the-top hedonism-overdosed character he developed into. Only Ted really hits the ground running right from the pilot.
Judy, Marshall's mother, behaves normally toward Lily in her first appearance, but becomes the mother-in-law from hell eventually. Possibly justified if it's due to Lily breaking off the engagement at one point.
Mork from Ork in his first appearance on Happy Days was a far cry from the cute and cuddlyinnocent pacifist he became in the spinoff. His mission was to collect Richie as a specimen, and he would freeze people at the slightest provocation, and his final trick to defeat The Fonz in the "hollytacker" would have been to put his two fingers together and make Arnold's Diner collapse (he mentions he has even killed a few people doing this before!). He was, for lack of a better term, a complete and utter dick. And for the spinoff to work, they had to change that.
A final scene was added later for syndication where Mork reports to Orson about moving from the 1950s to the late 70s, the time setting of his new show.
In Frasier, the characters of Frasier and Roz hit the ground running (with Kelsey Grammer in particular having had an unprecedented nine years of character prep), but Niles, Martin and Daphne change markedly over the course of the first season. Indeed, it isn't until arguably the episode "A Mid-Winter Night's Dream", towards the end of the first season, that Niles finally begins to acquire some depth.
For example, Niles was originally . . .well, there's a reason Replacement Flat Characterused to be called The Niles: he was a Flanderized copy of the uptight, prissy, snobbish Frasier of Cheers. By season two, it was established that while he was more rigid and neurotic than Frasier, he was less arrogant and insensitive and acquired completely independent personality traits such as overeager cheer and insecure Woobieness, among other sharp differences between his and Frasier's personalities. Also, Daphne was far more innocent and more of a Cloud Cuckoo Lander in the first season, as well as more stereotypically English, whereas she became more hot-tempered, snarky, and down to earth in later seasons' and Martin, who started out as something of a bitter, grumpy old has-been transmuted into a cheerful old slob who never let anything bother him. Most impressively, all these changes made the characters more nebulous rather than less, a rarity in sitcoms.
In Martin's case, his curmudgeonly personality can be explained by the fact that he had recently been forced into retirement after being shot, and his relationship with both his sons had been strained for years. Frasier had even told his friends at Cheers that his father was dead.
While Colonel Tigh'sincreased competence can be explained through story-related reasons, an early season 1 episode has him going to Roslin (whom he's known for no more than a month) to make sure Adama isn't putting the fleet in jeopardy over one missing pilot. By season 4, however, Tigh is known above all else for his borderline-absurd loyalty to his friends. Most people would agree it's worth it.
Kelly Kapoor in The Office (US) was first a normal worker, created only to be a foil for Michael's racist jokes. Soon after she evolved into a bubbly and adolescent ditz, a very drastic change.
Thanks to an extreme hair and fashion change between season one and two, she also managed to look 20 years younger.
You could make a case that nearly every character that is not part of the lead quartet characters (Michael, Jim, Pam, Dwight) could fall here. Even Andy started out as an unsympathetic Dwight-esque antagonist, and now he's one of the most likeable characters on the show.
New receptionist Erin went from being able to say John Denver's "Country Roads" is one of her favorite songs in her first episode, to not knowing who Snoopy and Woodstock and Marlon Brando are a couple of episodes later.
When Robot Girl Cameron on Terminator: The Sarah Connor Chronicles first meets John Connor in the first episode she is able to act like a normal high school girl and completely fool him. Right after she reveals herself to be a terminator, she completely forgets how to act human around people for the rest of the series.
In season 1 of Night Court, Dan Fielding starts out as a stuffy, pipe smoking prosecutor. By the second season, he became the sex obsessed lech and was that way for the rest of the series.
In the first few episodes of Gilmore Girls, Sookie is an ungodly klutz who lit her expensive stove on fire and nearly chops her own fingers off. By the mid-first season, this is gone.
Shades of this are seen throughout the first few seasons, but its not as exemplified like the first few episodes.
Luke in the pilot and a bit of the first season was large health nut (*serves fries* "These will kill you"; also begging Lorelei to not drink coffee), but this was more or less dropped by the end of that season in favor of things like fighting with Taylor and actually running a diner where he serves the kind of food that will "kill you".
Of course, his "health nut" attributes could be attributed to the fact that he's always been really protective of the girls.
In Ellen, a friend named Audrey has the job of annoying the title character all the way through...except that in her early appearances, she does this by being extremely pessimistic, but at some point she flips and is depicted as being obnoxiously perky for the majority of the series.
In the first episode Joey has an average intelligence, and in later episodes he is presented as Book Dumb. It isn't until the next season that he becomes through Flanderization the absolute idiot he is known as. This came to be a real problem when they attempted to spin him off into his own show.
Phoebe started off as a well-meaning, Cloud Cuckoolander character, complete with cleansing auras. By the later seasons she had become a lot harsher and more selfish especially to Ross and Chandler. Her high sex drive also wasn't really present in the first couple of seasons.
Chandler is remembered for being frequently Mistaken for Gay. In the first season it was funny because he clearly wasn't - he liked beer and Baywatch as much as Joey. Later seasons gave him effeminate quirks that made him being mistaken for gay kind of understandable.
S Club 7 were all rather different from their more familiar personalities in their first TV show Miami 7. By the time LA 7 came around, they had all settled into their more familiar personas:
Tina started off as something of a Team Mom figure. She morphed into a more bossy and pushy character.
Paul was rather normal if a little unlucky. He became more of a Man Child.
Jo started off as a bit of a tough cookie but eventually morphed into a full-on Lad Ette.
Rachel was more ditzy and obsessed with her looks. Later on she became a Girl Next Door who was more innocent than actually stupid.
Bradley was always a bit of a ditz but developed Serial Romeo tendencies.
Jon started out with pretty much no personality but became a bit of a Pretty Boy and frequently took on the Only Sane Man role.
Hannah underwent real Flanderisation and became a Genki GirlDumb Blonde. She's much less quirky when the series begins.
The cinephile Tony from later seasons of NCIS would be appalled by the Tony of the first season, who misses several classic film references and who never saw Shane or The Maltese Falcon.
More glaringly, in the pilot of NCIS, Agent Tobias Fornell of the FBI doesn't recognize Gibbs as he enters Air Force One, to the point of being surprised when he learns 'those agents' were NCIS and not the local coroner. Later episodes establish that not only are Gibbs and Fornell old friends, Gibbs warned Fornell not to marry one of his ex-wives. This was a conscious decision rather than a mistake, the creators loved the chemistry between Gibbs and Fornell so much they decided to give them a past and make Fornell a recurring character.
According to later seasons, McGee was a Weebelos scout and retains outdoors skills from those days, which makes his falling victim to poison ivy in season 1 or 2 a little odd (but still hilarious).
On CSI: Crime Scene Investigation, Grissom is less of an introvert and stoic in season 1. He's even prone to bouts of anger, such as when Ecklie has a crime scene cleared before Grissom had a chance to re-visit it (prompting Grissom to slap a glass coffee pot out of Ecklie's hands). He shields himself more as the show goes on, for good in Season 3 when he starts to battle his hearing problem.
In the first two episodes of Robin Hood, Marian advises Robin to let several prisoners hang so that he can "play the long game" and work the system from the inside out; on the other hand, Robin isn't prepared to let individuals die in favor of "the bigger picture". By season two, their standpoints have been completely switched around.
In early episodes of 30 Rock, Jenna was somewhat neurotic and flirty, but otherwise mostly normal. Now she's an over-the-top Attention Whore. When Jenna gets bumped from Late Night in favor of Tracy in an early episode, her response is to cry, saying that "I just feel like everything's always taken away from me." This would be pretty out of character for latter-seasons Jenna because (a) she wouldn't feel the need to justify her feelings as she would consider them to be justified by default, and (b) she would be more likely to respond to something like this by acting out rather than crying.
In the first season episode "Blind Date", it's revealed that Liz has won an Emmy. This is pretty unlikely based on later episodes.
The pilot episode contains a surprisingly somber (at least at first) scene where Tracy shows Liz the poor neighborhood where he grew up and reflects on his troubled childhood. A later episode establishes that Tracy remembers absolutely nothing about his youth, as he had blocked out all of his traumatic memories. He is even reduced to tears after being confronted with those painful memories for (supposedly) the first time in years.
In Whose Line Is It Anyway?, it can actually be quite surprising to see peoples' early performances - Even Colin Mochrie and Ryan Stiles were pretty timid during their early performances. Granted you'd have to go very far back to the UK version to see how timid they got, but you can perhaps spot the biggest improvement in Kathy Greenwood. She seemed to sort of be there for some scenes during the show; but later on became quite good at personalities.
It's actually quite amazing when you see Ryan Stiles's first performances on the UK Version.
The early episodes of Seinfeld often have George advising a more na´ve Jerry on the little particularities of life that are relevant to their situation. These roles would be reversed through most of the show's run. The four main characters (with the possible exception of Kramer) also developed into Comedic Sociopaths as the show found its voice: their early-season incarnations come across as far more sympathetic and self-aware than they do throughout most of the series.
Jason Alexander also had his own "click" moment similar to that of Leonard Nimoy: initially he thought George Costanza was based on Woody Allen and played him as such until one day he walked up to Larry David claiming to be unable to make heads or tails of one of George's antics seeing as "not only could this never happen but no human being would react like this". David explained it happened to him and this was how he reacted. It was then he realized George was David's Author Avatar all along.
The first few episodes have Summer from The O.C. as a vapid, party-girl who drops her unconscious friend off at her front door, but is changed to a more likable person after her character became part of the cast.
The Big Bang Theory has a good case of this. Sheldon started as a less assertive/more arrogant version of Leonard who was nervous around Penny and seemed to compete for her attention (at least regarding their white boards with math equations on them). The Tag at the end even had him comment with a great degree of social insight concerning Leonards chances with Penny. A couple of episodes in and he had evolved into the asexual narcissist with No Social Skills whom we all love to hate.
Penny changed as well — the first episode establishes her as rather ditzy with low intelligence (she's a vegetarian who eats steak) and very randomly emotional as a contrast to the guys being science minded, but those elements faded as the show focused on her being a more normal person around the geeks. Penny's apparent change in personality can be attributed to being uncomfortable around the guys at first while trying to be nice and make a good impression nonetheless.
The creator himself said to just skip the first few episodes, it took them a while to figure out what they wanted to do with the characters. Specifically about episode 5, where Sheldon's complete ignorance of social issues and Penny's sly knowledge about it took hold.
Amy Farrah-Fowler began as a female Sheldon, stipulating in their first date (one she was only on because her mother forced her) that there was to be no physical contact (up to, and including, coitus). In later episodes she almost sexually preys on Sheldon. This is explained as her budding friendship with Penny opening her up to larger social experiences, which is an interesting counterpoint to both Penny and Leonard doing the same thing with Sheldon but at a MUCH slower rate.
Bernadette's early episodes had her primary characteristic being that she did not understand anyone else's jokes, later admitting that she sometimes laughs just to make Howard happy. This almost disappeared completely after her third episode and she gets the jokes just as well as anyone else, with her personality now more along the lines of adorably sweet but ready to snap if provoked.
Uther from Merlin has always hated magic, but in the first few episodes he would react to it by steadfastly denying its presence; these days he takes the slightest hint of its presence and immediately becomes paranoid (even when there's a rational explanation).
Ensemble Dark Horse Sir Leon had a few appearances in the second season before becoming something of a series regular in season three; one of his first appearances involves him violently tearing apart Gaius's study in the search for evidence of magic. The sight of him smashing bottles and ripping down tapestries is completely at odds with the gentler character of later episodes.
Another Uther example — In Season One, Uther wraps his hands around Morgana's neck and makes it very clear that the only reason she's still alive is because of the promise he made to her father. Then, suddenly, in seasons 2 and 3, he seems to love Morgana more than his own son. Then it turns out that he is Morgana's father.
Arthur and Morgana spent season 1 as Flirty Step Siblings. This trait had vanished completely by season 2 to make room for the Arthur/Gwen pairing.
In the earliest episodes of The New Adventures of Old Christine, Old Christine is initially portrayed as a little dippy, but a reasonably competent parent and business owner with a modicum of empathy towards others. Before long, she morphed into a completely ignorant, bi-polar narcissist who all but abandons day-to-day running of the gym to new partner Barb, and quite often doesn't even know where her own son is.
The first paintball episode of Community shows Chang as a badass gunslinger in the John Woo mode. By the time the second paintball war rolls around, he's suddenly a pathetic coward who never even picks up a gun.
Chang as a whole changed from a JerkassSadist Teacher in the first season to a Butt Monkey in the second season. After getting fired from his teaching job at the end of the first season, he enrolled in the school as a student and was suddenly the biggest loser in the world, and one who gradually became more out-and-out psychotic, at that.
As with most Pilots, watching the pilot after watching the rest of the series allows for some striking contrasts — however, possibly the character who's marched on most when compared to her pilot / season one self is Britta Perry. In the early episodes, she's often little more than the Straight ManLove Interest for Jeff Winger, with little indication of the wackier and more goofily neurotic character she would later become. This may be a result of Gillian Jacobs' personality affecting the character.
Particularly odd is the first season episode "The Politics of Human Sexuality", which featured Annie being very squeamish about sex and ended with her being unable to overcome being sexually repressed. In later episodes, she's a Covert Pervert and her prudishness is clearly an act.
In the pilot episode, Annie was much meaner and uptight, to the point that on the commentary track, Dan Harmon points out that she could be considered the villain of the episode (Jeff was also quite the Villain Protagonist in the pilot, since this was before his Character Development).
Speaking of Britta and Annie, Britta was initially the more mature and composed Straight Woman, while Annie was the more naive egotist with Cloud Cuckoo Lander tendencies. By season 3, they almost seem to have switched roles.
Troy started out as a mix between the Dumb Jock and Jerk Jock obsessed with Football. He quickly became one of the nerdiest and most eccentric characters in the cast, after forming a Heterosexual Life Partner relationship with Abed.
The first season of The Fresh Prince of Bel-Air had Carlton Banks as a pompous Smug Snake rival to Will. Starting in season 2, after being a Butt Monkey and having emotional breakdowns, he became the lovable nerdy goofball we all know today. Likewise, in the first season, Hillary is portrayed as a shallow, spoiled, socialite and environmental activist. Since season two she became just shallow and spoiled.
Temperance Brennen from Bones starts off as sassy, sarcastic, and socially adept. It didn't take long for her to become the walking stereotype she is now, and flashbacks to earlier in the series completely ignore the way she behaved in the pilot.
In the pilot, Zack just seems to be a quirky intern, and shows little to no signs of having any major personality defects outside of using morbid nicknames for the deceased in order to distance himself from the harsh reality of his work. His personality is pretty well solidified by A Boy in a Tree however, which makes a point to highlight his social and sexual awkwardness.
In his first appearances, Sweets is a little creepy, and pulls some nasty tricks on the main cast—possibly because he was a Gormorgan red herring. Now, he's a trusted and dear friend of all the main characters.
Hodgins is also subject to this - the first season sets Hodgins up as the show's resident Jerkass and Millionaire Playboy with serious anger control issues whose paranoia was used mostly for comic relief. As the show progresses, it's quickly shown that he's actually a Jerk With a Heart of Gold with a bit of a Big Brother Complex towards later Squinterns, and who uses paranoia and anger as a way to cope with dissatisfaction in his life and who actively rebels against his privileged upbringing.
In her first few appearances in Glee Britanny's facial expressions give the impression that she's just as devious as her cheerleading cohorts. It isn't until later episode that she ends up being The Ditz and the resident Cloud Cuckoo Lander.
Bulk and Skull, the Plucky Comic Relief of Power Rangers, originally started out as school bullies and they even got into fights with the rangers' alter-egos. As the show went on, this aspect of the duo was downplayed and they became more like class clowns that were capable of acts of heroism at times. They were eventually treated more like friends to the rangers and they even became junior police officers.
The Muppet Sweetums first appeared in The Frog Prince as an ironically-named vicious frog-eating ogre serving the Big Bad. It's hard to imagine Sweetums in this role now — from The Muppet Show onward, although still looking ferocious, the name has been wholly deserved.
Emmerdale's Ashley Thomas was, for the most part, a fairly likeable and well-intentioned country vicar, albeit with a few foibles, until he suddenly turned into a father-beater.
In the first episode of Sex and the City, when Carrie mentions the idea of "having sex like a man", Charlotte without batting an eyelash asks "You mean with strap-ons?" As she evolved into more of a priss who talks far more euphemistically than the other three, it sounds weird to hear her say so so blithely.
In the first season of Boardwalk Empire, when trying to find an elusive criminal band of brothers, Richard Harrow suggests killing their innocent family members to flush them out. In the third season, the same Richard Harrow kills Manny Horovitz for killing Angela Darmondy When Nucky Thompson, the killer of Angela's husband Jimmy, finds out, he tentatively asks Richard why did he avenge Angela and not Jimmy. Richard says that while Jimmy was a soldier who fought in a war and simply lost, Angela was simply an innocent civilian.
The supporting cast of 24 all had markedly different personalities from their portrayals in the second season and beyond:
Aaron Pierce starts out as an extremely by-the-book field agent, whose only real purpose is giving exposition between major scenes. Starting in season two, he becomes a staunchly loyal (and badass) agent who often disobeys orders to help characters like Palmer and Jack.
In season 1, Kim Bauer is a rebellious teenager who (at the beginning of the season) doesn't trust her father, knows enough to understand when she's in a dangerous situation, and escapes from her captors on multiple occasions. Come season two, she's a Damsel in Distress who often has to be rescued by other characters and unknowingly gets involved in worse and worse situations.
Mandy the assassin is a cold, emotionless assassin in her appearances in seasons two and four, a far cry from her story in the first season, where she acts much more human, openly displaying emotion to several like-minded antagonists.
Season one of Sons of Anarchy had Bobby as a bomb expert as well as Opie. Sometime during either this or the following episode he makes a claim that his "big Jew brain can barely count his fingers." Fast forward to mid season and he's now the club bookkeeper and has never been tasked with blowing anything up since.
On That '70s Show, Eric turns into a Star Wars fan who is a huge geek about it; and the other characters easily get tired of his habit of constantly making irrelevant references to the movies. However, an early episode has the characters watching Star Wars, and all of them like it, with no hint that Eric is especially enthralled by it or that a character like Hyde, Donna, or Jackie would be embarrassed about it or feel that "only a geek would like it" (certainly their stance in the later episodes, when making fun of Eric).
Xena as she debuted in her original trilogy of episodes in Hercules: The Legendary Journeys went through different stages of her character that didn't really resemble her when she got her own show. In her first appearance she was The Vamp using sex to gain her soldiers loyalty and even sending them to their death if it suited her plans. By her second appearance all aspects of The Vamp were removed, no hint was made that she was sleeping with her soldiers, and she had a code of honor that seemed contradictory with the character we first met. In her final episode before getting her own series she was more or less just Hercules Distaff Counterpart. The Xena of her own show most resembles the 2nd interpretation.
Criminal Minds features this quite a bit with its revolving door of writers:
In the later seasons, Dr. Spencer Reid develops what Garcia calls an "anti-technology quirk," in spite of scenes in the earlier seasons where he analyzed subjects' computers in the field. This turns into Fridge Horror when you realize that the last time he displays a tendency for technology is in a two-part episode where he's kidnapped by an UnSub who uses computers to stalk victims and record his kills.
The book "Empty Planet"- about how automated systems could turn into robots and destroy the world- didn't help either.
In "Won't Get Fooled Again", Derek Morgan is established as the team's bomb specialist, even educating Garcia on the composition of the UnSub's bomb. However, later on in the series, Morgan is never again called upon to look at a bomb, even when situations would require a bomb expert. The worst examples are "The Fisher King Part 1" (when the team all received strange packages), "Empty Planet" (which featured a "personal cause" bomber similar to "Won't Get Fooled Again"), and "Run", where Prentiss is the one defusing the bomb.
Deadwood: Richardson's first appearance has him squeeze Silas for a bribe before giving him some basic information. Later appearances establish Richardson as an idiot Man Child without the cunning or inclination for such schemes.
Since The Real Housewives tracks the development of relationships, changes often take place as more is revealed about the cast members. Camille of Beverly Hills began as strongly disliked, but gained a lot of popularity during her second season; New Jersey's Teresa, on the other hand, showed a less likable side of herself after her sister-in-law joined the cast, and then an even less likable side after her legal problems came to light.