Common Reasons for Introducing One
- It requires a personality trait that no one else has: For instance, if the Aesop is "Don't be a Lazy Bum", yet no one on the show is lazy, you'll need to introduce a new, lazy character or else fall into the trap of giving characters flaws that weren't there before.
- It would change a character too much: For example, if a Drugs Are Bad lesson involves addiction, you can't have an established character get addicted and then go back to normal by the end of the episode, because that's not how addiction works. Likewise, if something happened that was serious enough to cause trauma that doesn't go away after a short time, such as PTSD, there's really no other way out— Having the character bounce back could carry
Unfortunate Implications, while having the character stay traumatised may darken the show's tone too much, so introducing a new character is the best option. This also applies to less serious matters, such as teaching a lesson about divorce without breaking up any of the established couples. See also Status Quo Is God. - It requires a member of a group, and none of the characters belong to said group: Very common for the Prejudice Aesop. You can't, for instance, teach a lesson about racism when all of the characters have the same ethnicity without introducing someone with a different ethnicity. Anti-ableism stories could have someone becoming disabled, but unless it's the kind of disability that can go away, you run into the "status quo" problem mentioned above, and, of course, some disabilities are inborn. The Gay Aesop can be executed without adding a new character, but then you'd have to make sure the character you're outing hasn't had any opposite-sex romances unless it's actually a Bi Aesop.
- It needs someone to die: For instance, if the episode is a Very Special Episode about how Death Is a Sad Thing, or about how dangerous driving can kill, and you don't want to kill off a recurring character, what else are you to do?
- It involves shady behaviour that the fans may complain about: Sometimes, the moral of the story has to involve someone doing something that, while it may not clash with their personality like in the first point, is still something the fans wouldn't want to imagine their favourite characters doing. For example, an anti-ableism episode that involves recurring disabled character Alice meeting recurring character Bob and finding out that Bob is ableist might make sense in terms of characterisation, but Bob fans might not like the idea of a character they previously liked doing something that bad. Therefore, a new antagonist might be introduced.
Potential Reasons for the Character Never Returning
- The writers don't know what to do with the character now: A very common out-of-universe reason is that the writers just don't know how to use the character in future episodes. After all, they only designed the character with this moral in mind.
- The character is too dark for the work: For instance, if they have a serious issue (like the aforementioned PTSD) or if they're a little too evil to be a recurring antagonist. After all, the viewers might be fine with an occasional Unexpectedly Dark Episode, but not want the show itself to become darker.
- The character died: Because obviously, you can't return to the show if you're dead.
