The longer the story, the more failures there should be and the greater the change that should occur.
This is the case for anything you write, but the more episodic the series is, the more this holds true. TV series, ongoing web series, and web comics are the most obvious examples of this.
Basically what this means is that your characters can’t succeed at everything they try to do. One thing about shows like Supernatural (the early seasons) is that you as the viewer know that, for the most part, by the end of every episode, the Monster of the Week will have been defeated and everyone you care about will still be alive and healthy. There are overarching plots, but they are tangential to most episodes and don’t affect much.
In Stargate SG-1, on the other hand, they spend eight season facing one major enemy (the Goa’uld), and they spend many of the episodes fighting the Goa’uld in some form or another. And sometimes they fail and the Goa’uld win, and sometimes they win and that later helps the Goa’uld win, and sometimes they don’t fight the Goa’uld at all, and those missions may be either successful or not to a lesser degree. Beyond that, there are lower level failures: they try to make a spaceship and it almost kills some of them, they try to make a new spaceship, it doesn’t work as hoped at a pivotal moment and they almost lose the entire planet, they build a giant spaceship and it gets stolen (briefly), they build more giant spaceships and one gets shot down over a planet and then later they need to get that spaceship home and it (temporarily) gets stuck in a giant sentient gas cloud. All of this means that sometimes they don’t have a spaceship that can do what they need even though they’ve been trying to build one for most of the show, but at the end of the show, they end up with spaceships whose capabilities and weaknesses play a pivotal role in the show.*
My point in recounting all of that (other than to get you all to watch Stargate) is to show that, especially when you have a long series where you want to show a great deal of growth (and I’ll explain why you need that in a second), you can’t just have them win every time they try to grow or every time they try to defeat an enemy. You have to have them fail, too, or there will be no stakes and it will be hard to suspend disbelief.
So…why do you need growth?
Basically, if you end up in the same place that you started, what was the point of your story?
Well, you cry, they defeated the major enemy. Isn’t that enough?
And to that I ask (because I like holding imaginary teaching sessions): If they could defeat the major enemy (or if they could get the girl/boy/non-binary person, or if they could get into the school they wanted, or if they could do whatever else they want to do) with the capabilities they had in the beginning, why didn’t they? There is no need for a story if your characters have everything they need to succeed when the story starts.
And as for why you need failure? Here are three reasons.
One, failure is realistic. Things rarely work well on the first try, especially more than once, which means that the more things a character (or group, organization, etc.) is trying, the more they should fail. If you think about someone trying to learn a language, they basically never (without an eidetic memory) remember all words the first time they see/hear them, or use grammar perfectly on the first try, or pronounce every word correctly. They will get some, but they will rarely get all. The same should go for someone who is trying to learn how to fight, for example. Even if you get everything right the first time you are shown it (which may or may not happen), you’re not going to get it right every time. You might fail more at some things than at others, or fail at the same thing over and over. Sometimes it’s because you don’t understand how to do it, sometimes it’s because your brain and your body aren’t communicating well, and sometimes it’s because your muscles just aren’t strong enough or your body isn’t flexible enough for it to work. Those are all failures that can and do happen in real life.
Two, failure raises the stakes. If you know the main characters are going to succeed at everything they try, or that their failures aren’t going to have any consequences beyond that episode (or chapter, etc.), there are no stakes. There is no concern for whether the character will do well or whether they will be ready in time, because they always are. There is no risk, because there is no failure.
Three, failure is interesting. As we see in Stargate, entire episodes can be built around failures. Failures make for interesting storylines, and sometimes successes that turn into failures can turn into even more interesting storylines. You defeat the Big Bad only to have a Bigger Bad rise up because of it? That’s a great storyline, and shows what was ultimately a failure by the characters. You stop someone for personal reasons at the expense of stopping someone for strategic reasons? Great storyline, because it not only prolongs and changes the conflict, it also adds an opportunity for personal growth and/or conflict into the mix.
With that, failures can also cause really interesting interpersonal interactions. Let’s so all of the characters are counting on Bob to pull off one part of the plan, and despite trying his best, Bob fails. Now everyone blames Bob (or maybe some subset of them blame Bob, depending on their personalities) and it causes tension in the group. Maybe this tension ultimately leads to Bob leaving because he can’t take the blame anymore. Now you have a splintered group all from Bob’s one failure.
What types of growth and failure can you have?
(I’m glad you asked, me.)
Here are some examples (primarily for militaristic/adventure type stories, but there’s a mix)**:
Building an army (or a group of people)
Not be able to convince people to join
Have traitors in the midst
Have large numbers die/be killed
Have ideological/strategic differences with allies
Building a new form a transportation
Not have it ready in time
Have it not go far enough
Have it not go fast enough
Have it explode mid-journey
Not have it ready in time
Have it explode in testing
Not have the magical capacity
Not understand the theory
Perform key rituals wrong
Be unable to pronounce words
Be unable to understand spoken words
Translating/decoding something
Have someone else find it first
Not have sufficient forces
Not have sufficient ability to break walls
Be unable to hold territory
Getting a romantic partner
Make bad decisions while intoxicated
Forget significant dates/events
Say inappropriate or mean things
Misunderstand what is being said
Not getting good enough grades
Having other life issues that distract from it
Be unable to create a working organizational structure
Be unable to create adequate civil service (police, roads, etc.)
*Of course, Stargate has some of its own issues with this, like the fact that Daniel has been brought back to life more than once, so the viewers stop believing that Daniel is ever actually dead.
**When I use the term failure, I don’t mean that it is the fault of the character or organization (necessarily, though in some cases it might be). I just mean that it is not-success.
