Randy Marsh's drift into Mr. Serious Business in the later seasons makes early episodes strange to watch; apparently World of Warcraft and Little League games deserve more panic than things like spontaneous combustion or lava engulfing the town. This is especially notable in "Spookyfish", where he actually plays straight man to Sharon's antics.
In "Two Guys Naked in a Hot Tub", Stan refuses to hang out with Butters and Pip because they are "Melvins" and the geekiest kids in school. Whilst Butters and Pip are still Butt-Monkey types in later seasons, Stan evolves into one of the nicest and most tolerant characters, while Butters practically becomes one of the gang, so Stan acting like he wouldn't be caught dead with them seems somewhat unusual. Even Kyle agrees with this point of view at the end.
Stan and Kyle actually were a lot nastier and more immature in the first couple of seasons, and actually seemed like genuine 8 year olds a lot of the time. It wasn't until around Seasons 6 and 7 that they became the eternal Straight Men and Only Sane Men to the idiocy of the wider South Park population. The change is most noticeable in their treatment of other kids such as Butters. Originally, Stan and Kyle were manipulative bullies only lower on the scale than Cartman; for example, in "Jared Has Aides", all of the boys are seen bullying and exploiting Butters in an equal fashion. In subsequent episodes, despite getting thrown out of the gang, Butters seems to gain a more developed relationship with them as their more righteous tones take over. As early as in "Toilet Paper", which aired a season later, they have resentment to exploiting Butters in a similar manner than they did before. Their nastier side occasionally returns, but they seem more ineffectual about it (cases such as "Pandemic" and "The Tale of Scrotie McBoogerballs" where their manipulation backfires on them).
Butters' earlier behavior is almost completely unrecognizable to his behavior nowadays. In his earliest appearances, he was as much of a Jerkass as most of the other kids, and would gang up to pick on any other singled out boy.note Such as Clyde in "Cartman's Silly Hate Crime 2000" when he is referred to as the "second fattest" of the group. It's a far cry from the naive Nice Guy he quickly evolved into.
In the first four seasons, Eric Cartman was a whiny spoiled brat who begged his mother into getting whatever he wanted and picked on Kyle mainly because he didn't celebrate Christmas. Also, the others got along with him for the most part, his voice was raspier with a lisp in the first two seasons, and he had many catchphrases. From Season 5 onwards, he is more intelligent, conniving, manipulative, and psychopathic; his voice is lower and deeper; he rarely whines or cries; the other boys hate him and rarely get along with him; and he rarely says any of his catchphrases anymore. He also had a genuine tenderness when it came to his family; happily singing "Over the River and Thru The Woods" with his mom, and being genuinely upset about not knowing who his dad is.
Remember when Wendy Testaburger was once a psychotic Yandere who had Iraqi troops murder a substitute teacher Stan had a crush on? Compare that to the smart and sensible girl, who is even willing to break up with Stan, as seen in later episodes.
In the early episodes, Mayor McDaniels was a Mayor Pain, obsessed with her own image but so stupid that she needed her aides to correct her every time that she gave a statement. As time went on she got smarter as the rest of the adults got dumber; these days, she's either just the leader of their antics, or she's portrayed as an Only Sane Woman forced to deal with a town full of idiots.
The town itself experienced this. In the early seasons, it was a small conservative mountain town, with a homophobic populace, only one police officer, and rednecks Uncle Jimbo and Ned playing major roles in many episodes. In the middle and later seasons, the town grew significantly in size, featuring a Wall-Mart, a large community center, a two-story police station full of cops, a number of restaurants, among other things, Uncle Jimbo and Ned have largely been demoted to extras, and its politics have moved significantly to the left, to the point where it usually more closely resembles a Bourgeois Bohemian-filled liberal suburb than a conservative mountain town. This is currently Truth in Television throughout the United States, particularly in the West, as towns are growing larger and more "modern" thanks to the influx of migrants from the big cities. Trey Parker and Matt Stone are noted libertarian advocates, so their politics can swing from one Strawman Political to another pretty quickly.
Kenny's original role in the show was simplyto die a horrible deatheach episode. By Season 5, the creators got sick of the gag and decided to kill him off for real in "Kenny Dies". They revived him at the end of the following season. Since he has died less frequently in subsequent seasons, Kenny's role has evolved into The Hedonist.
Craig Tucker has gone through this twice. In the early seasons, his Running Gag was constantly flipping people off; if called out on this, he would blatantly claim that he didn't do it, even as he did it again. Around Seasons 7-8, this trait was almost entirely dropped and he got his own group of side-character friends who were The Rivals to the main boys, and sometimes even acted like bullies. After that he stopped hanging out with his gang and became more of a stoicDeadpan Snarker, a trait which was especially solidified in his day in the limelight two-parter, "Pandemic"; this proved immediately popular and has remained ever since.
