to be more like the actors or actresses playing them? Basically, what TV Tropes has to say about supposed or alleged "Mean Characters, Nice Actors" (https://tvtropes.org/pmwiki/pmwiki.php/Main/MeanCharacterNiceActor) and vice versa (https://tvtropes.org/pmwiki/pmwiki.php/Main/NiceCharacterMeanActor). In other words, down the line it became less like we were seeing a multi-dimensional character as originally written and more like the actors or actresses were simply playing (https://tvtropes.org/pmwiki/pmwiki.php/Main/WagTheDirector) a close variant of real life personality. Or more specifically, longer the stars stayed with the series, the more that they tried to make the characters seem like they imagined themselves.

Blair Warner on The Facts of Life had been known to wear Daisy Dukes and low-cut blouses during the first season of the show, but in later years, Blair dressed as conservatively as Lisa Whelchel. Blair also went from being a girl interested in art and painting to someone who went to law school, and this this mirrored Lisa Whelchel's ambitions.

The classic example is the Andy Taylor character on the Andy Griffith Show. The original character was a version of his comedic persona from his early days as a stand-up, as the seasons went by Andy evolved into what we now understand as Griffith's real personality,
split between Angry Andy/Tranquill Taylor.

I have a hard time watching some of the season 1 episodes of The Andy Griffith Show due to his over-exaggerated accent and his attempt at being the funny guy. I'm glad that changed subsequently.

Andy Taylor/Griffith was complicit, mischievous, and more tolerant/bemused of Barney in the initial seasons but gradually became a grouchier foil/straight-man.

The same thing happed to Samantha Stephens on Bewitched. Sam was a naughty witch amused by her meddling family but by the late seasons was almost as crabby as Darrin.

To a lesser degree also Mary Richards/MTM - she became less naive and less tolerant of her funny friends and more an exasperated foil.

Serena went from cool to kooky.

Potsie went from nerd to nitwit.

Family Ties: Mallory and Stephen Keaton both started out pretty normal. Then they changed Mallory into an idiot and Stephen into the stereotype stupid dad.

Already mentioned, but Potsie on Happy Days was basically the same thing.