Plot Armor, this is a word used to describe just about any protagonist in just about any novel. It's the assumption that your main character is likely not going to die. They probably won't be horribly wounded either, in general. Plot armor can protect you from physical harm, but also mental and emotional trauma as well. And no writer in their right mind is going to take their main character and suddenly break them in the middle of the book. If your point of view character dies, how are you going to continue your story?
This doesn't necessarily mean that they might not get killed off near the end, but generally, a protagonist is protected from any significant harm. That's not abnormal. Most of us want to see our protagonist survive the entire story. Most of us also want our protagonist to have a happy ending. Most of us don't like surprises, especially when those surprises completely rearrange how the rest of the story is supposed to go. No one likes to get attached to a protagonist only to have someone play switcheroo on the main character.
However, the byproduct of this mentality is plot armor. You can't surprise us, because the reader knows that no matter how messed up things get, the protagonist will probably survive. How can you enjoy a scary story or a thriller, when you know your protagonist will make it to the end? How can you create suspense and dread, if the plot dictates the MC will have to come out on top? Maybe you can make the climax suspenseful, after all, killing off a character in the final book (Harry Potter anyone?) or killing off a character at the end of the story is still viable... until that story comes out with a sequel, and a sequel to the sequel, and then everyone reading your story knows the character has to survive to be on the cover of the next book.
While reading Harry Potter and the Goblet of Fire, at no point are you ever left wondering if Harry will get out of it. He has plot armor a mile thick, and there isn't a chance he's going to get hurt until at least that final book. The reader knows it. The writer knows it. So the question becomes, how do you create emotion, worry, suspense, and anticipation when we know from the get go that some characters are always going to be alright?
Plot armor has become so prevalent in fiction that stories that lose even a little bit of that plot armor (Game of Thrones, The Walking Dead) are heralded for their open stance on "anything goes". However, even these shows are always slathered in plot armor. In Game of Thrones (book, not tv show), being a stark seems to be pretty safe. Most of the characters that are always dying aren't main characters. With the exception of a certain wedding, I can't say there was a single "surprising" death in that entire series. The plot armor is so thick that even after a certain character dies in the last book, no one believed for a second he wasn't coming back. He was so instrumental to the way the plot was set up, his psych out death barely even phased anyone. Then look at walking dead (the comics). Lots of people die, but even a comic/show known for its willingness to kill ANYONE still has the Carl and Rick alive and well. For that matter, the TV execs know they'll get a riot if they even think of killing Daryl, so his plot armor might as well be made of adamantium.
So do I have an answer on how to deal with plot armor? Not really. I have ideas on how to minimize plot armor. I also must go on record and say that a lot of people don't like stories without plot armor. I like to know that my main character is going to survive alright. I don't like the thought of reading characters I like brutally die one after another. That's just depressing to read. There are plenty of stories where plot armor is the best part. So here are my five tips and explanations on Plot Armor, but the first thing you need to understand is that...
Every story doesn't need to be about people dying
Quick, tell me a story from your life. Any story of something memorable that happened to you. Did it involve someone dying? We all might have one or two stories when we lost a loved one, and when we get older, we might have a few more stories, but in general, I'm betting most of the stories you could tell don't involve the loss of someone close to you.
However, we seem to build this opinion that death in stories, especially stories with action, drama, or excitement, needs to have a few characters die. On the particularly long stories, you know, the epics... death almost becomes expected. You assume in the last book that some characters are going to die.
However, this isn't necessary. You don't need death to make a story interesting. You don't need death to make it suspenseful, and you don't even really need death to generate fear. Some scary movies don't depend on killing character after character, and yet can still be frightful just with the threat of death. Take the infamous Doctor Who episode "Blink". The Weeping Angels are terrifying monsters, and yet not a single person died.
I know, know, that's a cheap example. After all, not many bad guys use time travel as a weapon. Technically two people "lived to death" in this story, her friend and the cop. However, the point was still there. The scariest scenes, the ones with the greatest impact didn't involve a single death, or even threat of death.
So what does that mean for your story? Well, that's up to you. However, I'm telling you that there is more than one way to generate concern for your character. Threat of death is only one way. In essence, it isn't how close your character comes to death that's the suspense, it's how much your character has to struggle to achieve.
Seeing your character fight and struggle, and even sometimes fail, that's truly when kinks in plot armor are formed. Basically, the truest form of plot armor comes when you make a Mary sue, or the perfect hero... essentially those who are infallible. If you never make mistakes, then no matter how badly a situation gets, people assume it's always going to go the MCs way.
If you want to make a character with less plot armor, you need to make a character who fails. Maybe not in the case of the big things, but everything can't turn out their way. And I'm not talking about last minute twists that cause things to go outside their expectations. If you really want plot armor to go the way of the dodo, your character needs to make plans, and those plans need to fail.
Maybe they expected someone to appreciate a flower they sent, only to have it anger them more. Maybe they made a plan to sneak somewhere, only to get caught and held by police. Have them face the consequences. Have their plans fail, and don't give them some backup plan where they come out of it on top. Naturally, these failures need to be part of the plot. You can't just have them be a failure and then suddenly when the story starts they never make a mistake.
It's all about putting chinks in the armor, dent it, thin it, and hit it until no one even recognizes it as plot armor in the first place. Then you need to...
Plan ahead of time and stick to your decisions
Basically, I'm telling you not to throw in plot twists to keep things interesting. When you create your characters and put them in your story, it might be a good idea generally to decide what you want to happen to them. If you want someone to lose a hand, or lose an eye, plan that crap in advance. Think about where your character will start, what your character will experience, and where your character will end up.
Plan accordingly, because that lets you drive up the tension, suspense, and concern for your MC within the limits you prepared. Don't just suddenly do something for shock value. That will almost certainly anger your audience. That's not to say don't create a shocking scene, but make sure that scene makes sense within your narrative. Prepare for it, lead up to it, and by the time it happens there should have been some signs leading up to it. Maybe they had a conversation just ten chapters ago about their own mortalities, maybe they had a nightmare at the beginning of the book that turns out true... whatever it is, and the trauma you put a character through shouldn't come off as random. The best of us should see it coming, and worst of us should at least be able to nod and say "Yeah, that seemed necessary".
A good example? Game of Thrones, Eddard Stark. Sorry, spoilers. In the very first part of the book, the Stark family stumbles on a dead dire wolf (their family sign is a Direwolf). The dire wolf was a parent who was killed by a boar, and it had a pack of cubs that were still alive. Each cub was given to one of the children. At the end of the book, Eddard Stark is killed... his cubs succeeding him. And who was he killed by? The Barathean family, represented by a boar.
A lot of people still complement the first book for the "surprising" death of Eddard, but Eddard's death wasn't all that surprising, was it? It was heavily foreshadowed, and everything leading up to it seems obvious in retrospect. That's the power of planning ahead and not just throwing twists as you read. Throwing twists upsets a reader, planning a death is great.
However, when you plan something to happen to your character, don't just have them get hurt and then magically get better. If your character is put in a harsh circumstance, even though you "planned ahead" for him to get out of it, if the consequence of that is that he's right as rain, it will only affirm the plot armor you gave him. Therefore...
Always Show the Consequences
The consequence of breaking a bone is usually severe. A person who had been cut, and gouged, and injured is not going to heal up lickity split and be fine. Even in a world with magical healing, there ought to be at least some consequences for your actions. If healing can make someone instantly fine, then there will be very little weight to anything a character does, presuming they survive the next round of challenges.
Some suspense can be brought back by the concept of an instant death. I get this idea mostly from video games. What are some of the scariest video games of recent years? Outlast, maybe PT. What makes these stories so different than the Resident Evils and Silent Hills of the past? Sure, you have no weapons. Sure, you can't fight back. But also, there is the inclusion of the instant kill.
In the world of modern gaming, which has replaced HP bars with insta-healing for convenience, the only real way to generate franticness and fear is to make a character that can be killed instantly with the wrong step. That same concept can be applied to books. Instant failure can drive suspense better than any amount of harm you give a protagonist. And once again, this is NOT necessarily talking about death.
Instant failure can be brought up by failing to rescue someone else, perhaps having a conversation with someone and knowing the wrong words will set them off, or perhaps standing precariously over an edge, about to fall. Putting someone right at the cusp of complete devastation and complete success is a key to helping generate fear and concern for your character. Having them actually experience that devastation, at least once, will help keep plot armor at bay.
However, that all depends on your ability to show consequences. Their failures should affect their personality. They should be bothered by it. It should eat away at them. If those failures are on the physical side, they should be limited to what they can do, limping around and unable to do the same tasks they could easily accomplish earlier in the story.
It should take some time for them to repair, and that time should have consequences on their story. Don't have them break a rib, and then say "6 weeks later", and have him continue the story without a gap. That subverts the breaking of a plot armor. If you want that plot armor to crack, you need to show people that your characters are human, and will face the consequences of that.
No matter how much you beat up, bruise, slash, and damage your hero, if you want people to see him as anything but invincible, he needs to experience that pain, and a throw away like "It hurt a lot" followed a paragraph later with "Now I'm fine", is going to just aid in us seeing him as invincible. Unless your character suffers, the pain is meaningless. This is a discussion even scientist's struggle with. Does an insect feel pain? The answer is yes. They have a pain stimulus and they react to it. But does an insect suffer? That's the question. We don't care if an insect feels a stimulus to pain. They feel a stimulus to pleasure too. What we want to know is are they suffering by the experiments/poisons we use on them. We still haven't figured that one out. That same goes for your protagonist, if they don't suffer, they might as well not be in pain at all.
Make your reader think they are your main character
Naturally, the only way you can show suffering is to put your reader into the eyes of your main character. Your main character has to feel the consequences of their actions, and so does your reader. To truly break plot armor, you frustrated protagonist who can't do what they once could do, let's say because their leg was broken, should transfer their frustration over to your reader too. Your reader will shout, no, you got to run! But you can't run! Your leg is broken!
When you get to the point that the reader and the protagonist are on the same wave length, you're going a long way towards humanizing that character. However, it isn't easy. Things that frustrate your reader would definitely frustrate a writer. You just want your protagonist to get where they need to be. You don't want to write three pages about how they killed someone for the first time and how they need to cope with it.
In video games, they call that ludonarrative dissonance. It's a situation where, in the name of gameplay, certain logical behaviors need to be tossed out. A prime example of this is in Tomb Raider (the new one). Lara Croft is forced to kill someone, and it's this big emotional scene. Only to murder 2 more people a few minutes later, and go on a Rambo murder rampage the likes of which have never been seen for the remainder of the game.
This might happen in books, but where you can argue a video game has to make these changes to fit with the gameplay, a book mostly just makes these scenes for convenience. Your character only spends one sentence contemplating the fact they're a murderer now because you don't feel like spending pages upon pages writing about the weight of their decision to kill someone.
Instead, you jump right to it. However, jumping right to it is the very thing that makes people seem like they have plot armor. The more "human" and "lifelike" you make your protagonist respond to situations, the more into their minds your reader can associate, and thus the more risk we'll feel when they are in panicky or frightening situations. If you're character comes off like a Greek god, no one is ever going to worry about them, no matter how bad the situation is. If your character comes off like someone vulnerable, disturbed, and weak... that plot armor melts away. However, sometimes you can't always do something to your protagonist without upsetting your own story. So when all else fails...
Increase the number of victims, I mean characters
Put in more characters. Give them stories, give them personalities, and then kill them. This is not the same thing as a red shirt. Red shirts rarely have names or personalities. The trick is putting a character in there that people genuinely like. The clichéd way to do it is to either make them a family orientated person, or make them funny.
In the case of family orientated, they might come off like a mentor to the protagonist. Or, they might simply have a family of their own that they are always talking about. People always associate with the family man, and when he suddenly dies, we can feel the consequences of his death. He died, and now he has a family that has to fend for themselves.
The alternative method is to make the person funny. If the person is funny, then something as serious as death will resonate with readers. They'll remember him because he made them laugh. They might say something like "this guy was awesome". Thus, when the guy dies, we'll shout "Ah... I really liked that guy."
These are, I must reiterate, clichéd. More and more stories, from Game of Thrones, Walking Dead, Attack on Titan... they are all taking this method and refining it to a Tee. In some of these stories, it's like a revolving door. Introduce a group, get you to like them, kill them... introduce new group.
And remember, while I'm using the word "kill", since plot armor is often associated with literal life/death in stories, I'm not talking just about death. There is any number of things from physical injury to emotional injury. Even though you want your protagonist's relationship to be bulletproof, doesn't mean you can't have side characters who have rockier relationships. Just because your protagonist has come through unscathed, doesn't mean you can't have other characters having a much harder time coping with things. Adding characters to decimate them works.
I'm not saying this needs to be the case, but it's been proven in practice that a willingness to introduce characters and kill off anyone no matter how long they've been in the story is a way to obscure plot armor. As I pointed out in the beginning, the plot armor is still there. Certain characters will almost certainly survive to the end, but when all of their friends, including some of your favorites, suddenly die without warning, it becomes very hard to tell who is wearing plot armor, and who only seems like they're wearing plot armor. That gap can create a lot of tension, and keep readers guessing to the very end. Just make sure you plan ahead and you keep characters grounded.
However... at the end of it all...
Plot Armor is Okay too
There is no particular reason you HAVE to make all your characters seeming like they will suffer at any moment. I give a lot of examples, but there are plenty of stories where you want your characters in plot armor. Wish fulfillment stories, whether they be about an overpowered hero overwhelming his enemies or a couple getting together and forming a relationship, are all about plot armor heavy people unlikely to fail.
And... there is nothing wrong with that. So it's up to you decide how much plot armor your characters need or whether you need a story where readers think any character could die at any point. That'll all be just part of the planning process.