Let's start by giving a character--let's call him Joe--the trait of not liking bananas. It's early days, you're still figuring out how Joe ticks and what distinguishes him from your other main character (James), and that's an easy way to do it: James has already been established as being a sucker for banana splits. Now, fast forward 10 seasons: does Joe still hate bananas? Does James still make a point of ordering banana splits everywhere they go? It seems like long-running TV shows handle this kind of question one of three ways:
- Caricature: Joe's dislike of bananas is referenced somehow every other episode. James would do anything for a banana split, to the point of blowing off his job (and completely sidetracking what's supposed to be the episode's plot) just for the possibility of getting his hands on one. Possibly wacky hijinks ensue when the two characters' preferences collide, or there's recurring squabbling over this that's supposed to add tension but is really just annoying.
- Throwaway: These preferences are referenced once or twice and then completely disappear in subsequent episodes, to the point of even being reversed somewhere down the line. This is similar to the "hobby/job of the episode!" syndrome, and makes it almost impossible to do any significant character development.
- Character-building: Instead of beating the banana thing to death or completely forgetting it, those preferences get tucked away somewhere they can be found again, and are pulled out only when relevant. For added bonus points, they shift or deepen with each reoccurence. Perhaps Joe is won over by James at some point. Perhaps James overdoses on banana splits, or finds some other ice cream confection he likes even better--as happens with real people as they age and experience new things. Perhaps we find out at some point that Joe's family was so poor that the only fruit he ever ate as a child was bananas, and now it carries all sorts of connotations he can't stand (or he just got absolutely sick of the taste). Or James has some really good memory connected with banana splits and eats them for the remembered emotional high as much as for the taste. (Or even--Joe has actually been trolling everyone all along and really doesn't care about bananas one way or the other.)
Notice that this third option is longer than the other two combined--I could have continued for another couple of paragraphs just listing more ways to spin how these characters interact with bananas. Obviously it's the best way to avoid your characters becoming mere cardboard cutouts. But it also carries some risk: because there's more to remember, there's also much more potential to outright contradict past canon instead of flowing smoothly from it. The writers need to be well acquainted with the characters and the smaller details of the show in order to avoid this, which can be tricky with high turnover. Couple that with how nit-picky fans can be, and it's easy to understand why so many shows--especially those on their 8th or 11th or 13th season default to options 1 & 2 instead. They're more limiting, but they're also easier and 'safer'.
This is why I've never been bothered in SPN by the "Grand Canyon gaffe", or Dean using the term WASP in season 1 but being confused by it in season 10. These sorts of things are the minor slip-ups caused by fallible memories, not by the writers giving up on writing living, growing characters. And as a fan, I can actually reconcile both those particular instances (and most of the rest) within canon very easily. The only time they affect my ability to enjoy the show is when I run into a whole bunch of people complaining about them as though they prove that the writers secretly hate the characters and show they're creating.
(And very tangentially off that--I know some people are really nervous about the changes in the writing staff. But if you look at their credentials, they're all coming off a combo of horror and mystery, which has me very hopeful that we'll be getting a turn to more MotW episodes and non-Heaven/Hell drama. Which would be nifty.)