When something happens in a fictional story, there are two ways it can be interpreted: either as a logical result of different factors within the universe, or as the author making it so. In essence, the latter is always going to be the case, but the author's choices can match up with what makes sense for the story. When it comes to believability, this is one of the most important aspects. Every other part of a setting - armor design, clothing, weapons, whatever - are believable in the sense that they are part of a logical setup within the story. In most cases, it's something like this: "I need to deal with [x]. I will [y]." It acknowledges a threat or impetus and moves to deal with it. It doesn't matter whether or not that threat is swords or spears or rain and sleet, it is part of a rational decision-making process.
For example, when it comes to armor design, there are certainly reasons that someone might choose not to wear armor. It's heavy (to an extent), it can be uncomfortable, and in some cases it might not adequately protect against the weapons that the character is up against. However, their reasons must be established. In many cases when a character forgoes armor, it's just because they don't wear armor. In some cases, it may be justified by "slowing the character down", but the tradeoff is rarely worth it in plausible terms. That's dependent on internal consistency, though - if a character is markedly faster without armor, then that's fine.
Reasons don't have to be good, either. There's plenty of ways for a character to forgo armor for reasons of pride or insanity. Berserkers did not wear armor, and it added to their intimidation factor - they did not fear death. Unfortunately, people tend to miss out on the other half of this, which is the fact that berserkers tended to die a lot. It is a weighted decision to not wear armor. The irresponsible aspect comes when the negative aspects of that decision never come into play.
This is hardly the only example, but it illustrates a necessary concept: attributing something to a character or setting, versus attributing something to the author. This manifests in other ways as well.
In the Bretonnia supplement for Warhammer Fantasy Roleplay, there is a preface warning that states the following: "Women in Bretonnia are second-class citizens, and many careers are only open to them if they pretend to be men. This is not a feature of Bretonnian society of which the author or Games Workshop approves, but women pretending to be men make interesting characters in a roleplaying game. If the sexism of Bretonnia makes you or your players uncomfortable, feel free to ignore it".
What was the purpose of this warning? Without it, people might have assumed that this fictional society was endorsed by the author, making them misogynist. This is one of the dangers of authorial influence: it is your world, and everything that happens is your design. In many cases, this is reflected as an automatic endorsement of everything that happens, and in many other cases, that is in fact actually the case: the world was created in order to appeal to the author's viewpoint. If something is not outright endorsed or despised by the characters within the setting, the reader may make their own judgments about the author's viewpoint.
For example, in Warhammer 40,000, the fact that only men may become Space Marines (because of the way the process works) is often attacked as being misogynist on the part of the authors, rather than something that makes logical sense in-universe (and even if it did, people would argue it should be suspended rather than allowed). However, on the other side, there are many instances where an event that would seem strange or unpleasant in real life is treated as being perfectly normal in a context where it shouldn't. Many unsettling moments are founded in this concept ("No, Captain America, everyone here in the unprejudiced modern world thinks that incest is great! Deal with it, loser!"). It comes down to moral dissonance: the author is in charge of the world, and thus can create "absolute" good and evil in a way that the reader might not agree with.
This is an issue with gore, as I've mentioned. Gore is something that can exist logically within a setting (what else do you expect to happen if a head is smashed by a sledgehammmer), but its depiction can create accusations that the author enjoys or supports it. In some cases, this is accurate - there are certainly plenty of artists who draw gore for the thrill they feel. In many others, though, the gore exists simply because that is what would rationally happen. Differentiating them is an issue of intent: is the author showing this to create disgust and sympathy, or because he or she thinks that violence is "totally wicked awesome"?
A logical system can support (or disguise) this. If it makes sense within the confines of the story, it's more acceptable than "it just happened". For example, the issue of authors "killing off characters" is less of an issue if it's established that death can come suddenly and often. If 9 characters die slow, lingering deaths with dramatic speeches and the 10th gets shot in the head and dies, it's going to feel jarring or weird. If an event feels logically established within the rules of the world, then it's "fair".
People aren't perfect, but they're generally rational (even if their reasons for making a decision are based on emotional responses). They have things that influence them and cause them to prefer certain decisions or approaches. Even if their reason is bad, it's still a reason. Moreover, they should be held to the standards of their decision-making.
Let's go back to armor design for a second. There are plenty of examples in history of people going without armor, either because they believed they were divinely protected, because they were proud and arrogant, or because they felt that heavy armor would slow them down. However, the fact needs to be established that they were not "forgoing" armor, they were sacrificing it. Armor is something that would have helped them. They went without it for one reason or another. Therefore, they lack the benefits that armor provides in exchange for this other thing. This is a rational setup.
Here is an irrational setup: "She's not wearing armor because the artist didn't give her any". The difference here is that the character's reason for not wearing armor is not established or touched upon. Instead, it relies on a meta-justification. There's no reason in-universe for the character not to wear armor, and I mean that literally: there's no reason. Not that there couldn't be a reason, but one isn't given.
A character's judgment can be impaired by all sorts of things. They could be emotionally unstable - angry or sad or afraid. They could be chemically impaired by alcohol or drugs or sedatives. They could simply be unaware of the actual nature of what they're doing, or are generally inexperienced with it. These are all reasons for a bad decision being made. On the other hand, when a bad decision is made for no reason, it undermines the logic of the setting and feels like the author just needed the character to do something stupid.
Sometimes, players make decisions that don't make sense for a character. This is because of any number of differences in perspective - the player doesn't share the full range of the character's senses, the player doesn't have the same consequences as the character, the player isn't subject to moods like fear or terror in the same context as the character, and so on. The character isn't the player and vice versa, and thus the factors they include in their decision-making is based on differing information and priorities.
Sometimes, players make decisions that don't make sense for a setting. This is usually because of an abstraction in the rules - tactics and strategies that are meant to make sense in the lore or the fluff are not feasible in gameplay. There's two ways that this can be treated: either to adapt the rules to be more like the fluff, or to adapt the fluff to be more like the rules. Surprisingly, the latter seems to happen fairly rarely, although if anyone has any examples of an overpowered niche unit being acknowledged in-universe, I'd be interested in hearing about it.
Both of these things are decisions that make sense to the player, but not in-universe. They rely on logic, but not logic that makes sense to the characters. The character seems illogical because their reason makes no sense, but to the player it's perfectly rational. For example, the way armor works in many RPGs is enough of an abstraction that there's not really a reason to dress "sensibly" based on real-life logic. Instead, things like dexterity and evasion present a different picture of combat that simply would not pass in real warfare or in-universe warfare.
When an author is writing an event, there's a few questions to ask:
- "Why is this happening?"
- "Is it supported by the logic of the setting?"
- "Does the character's decision make sense in the context of the setting?"
- "Is the character's decision supported by their personality or state of mind?"
- "If these questions cannot be adequately answered, am I writing it just because I want to see it?"
A character's decision-making process reflects their viewpoint. A well-established process can make them feel like a real person with their own motivations, and thus is more believable. A poorly-established process makes them seem like a puppet of the author, and hence the reader will lose interest and have their suspension of disbelief broken.
So, To Sum Up:
1) When creating viewpoints and opinions in a setting, remember to differentiate between "your morals" and "the character's morals". Avoid the temptation to make them match up absolutely.
2) When writing a character, keep in mind the various factors that would influence their decision-making. The more it seems like "they" are deciding, the more believable it is.
3) When dealing with an abstracted concept, try to differentiate between what makes sense to the characters and what makes sense to players or other people dealing in meta-context.
4) Above all, provide some reason - even if it's not a good one, it's the character's decision. Don't forget, though, that decisions should have consequences.