The next symptom to step up and be judged is perhaps the most pervasive aspect of Mary-Suedom and of bad characterization in general: having characters with inconsistent personalities or whose actions don't fit what the narration is trying to tell us about their character. The antidote to this is, of course, to establish a realistic character and to take their actual personality into account while writing.
This chapter comes with massive plot spoilers for Howl's Moving Castle, by Diana Wynne Jones, The Chronicles of Narnia by C.S. Lewis, and The Dresden Files by Jim Butcher, the requisite technical terms, orange soda, and slightly less snark than the previous chapters. My snark is currently being taken on a walk by someone else, who hopefully knows not to let it eat Chihuahuas. They're terrible for its digestion.
Here's some pitfalls and how to avoid them:
Know what is, and is not, an actual personality trait: Every time I see someone list "beautiful," or "handsome" or "disabled" or "physically weak," as a character trait, I throw up a little bit inside my brain. First off, appearance has nothing to do with personality. A good character is relatable even if we know jack shit about their physical characteristics or their appearance.
In order for an adjective to actually be a personality trait, it has to be in the brain and determine the character's actions. To alter it would be to change the character on a person, at a fundamental level, forever. For example, you can have an extraordinarily beautiful character lose their beauty, whether by disfigurement or semi-permanent disguise or what have you, and it shouldn't, in and of itself, change their personality. For an example (actually, several examples in the same book,) I point you to Diana Wynne Jones' Howl's Moving Castle. (Not exactly the same as the Miyazaki film, sorry.) In it, Sophie is objectively very pretty and clever, but she has zero confidence since she's grown up as a sort of maternal-figure replacement for her two younger sisters, who are simply better at flaunting what abilities they have than her, since she's had to be more responsible and put off her own wishes. When she is transformed into an old woman (long story, trust me,) and simultaneously freed of her responsibilities, the story has nothing to do with losing her beauty and everything to do with her regaining her confidence and allowing her fierce, stubborn personality room to shine once more. In another example within the book, the two younger sisters trade places by means of a magical disguise that allows the younger daughter, Martha, the "smart one" to pass herself off as the middle daughter, Lettie, the "pretty one," and they essentially switch their roles in the story while retaining their original character. The relative beauty of Lettie matters very little when compared to her ambition, and though Martha's cleverness does matter in the story, her comparative lack of maturity and ambition prevented her from truly enjoying the situation (an apprenticeship to a witch) that her "cleverness" bought her early on in the novel.
Now, physical characteristics can affect a character's perception of themselves, but it always ties into a more stable part of their personality. For example, a character might not care about their appearance or physical strength, or not be in a context where it mattered at all. But on the other hand, they might be vain, and identify themselves with that shallow surface trait to the point where loosing it would be disastrous to them. But the beauty or strength is not the effective character trait – an almost identical plotline could be constructed where a musician proud of their playing ability lost a hand and couldn't play their instrument any more, sending them spiraling into a depression because they no longer have a purpose in life.
As a rule of thumb, if the trait affects other characters' first impressions of them before they open their mouth, it's not a personality trait. (This is a very rough rule of thumb – perhaps the character first met this person after witnessing them commit murder, and the cruelty or pure bloodthirstiness of their actions, which are a defining character trait, are the first thing anyone observes about them. But that takes showing the action rather than telling it, which we will cover later.) The second rule of thumb is that if the trait could be replaced by a different one and its effect on the plot would remain the same, it's not truly a character trait.
Use only real flaws: A real flaw is something that could conceivably prevent your character from reaching their goals, and should be a real personality trait. Being "so beautiful it's a curse," to quote the original Mary Sue litmus test, will not only get your character keel-hauled, but has nothing to do with your character's personality or the conflicts that they face and have to internally overcome. Neither does being physically weak or clumsy.
A real flaw is a personality aspect that doesn't always go over well. For example, perhaps you have a brave heroine. Fine, good. Now start looking for the logical extent of that trait, such as her being willing to go up against anything. Now, if you look at it objectively, when she is prepared to fight anything and risk anything, doesn't that make her a little reckless? Won't her companions get tired of her charging into situations, especially when they'd have been saved a lot of time and bother if she'd been willing to take a coward's route and walk away? Or perhaps people who care about her can't stand to see her get hurt when she goes up against things well out of her league? Or maybe she herself realizes that her bravery is getting her into situations that she can't get out of, and that she has to learn to accept outside help and her own limitations in order to succeed?
Perhaps you have a determined character; they could be so determined that they're stubborn. Or maybe your character who is sweet and shy is really meek and needs to learn to stand up for herself. Maybe your kindhearted character is stretching himself too thin trying to help everyone, or your badass, uncaring character has realized that no one trusts them any more because they've alienated all their friends and would-be supporters. Or maybe you have a sarcastic heroine – she's likely to get in trouble with parents and other authority figures for mouthing off, and being known as a smart-ass might actually hinder other people's ability to take her seriously. It's a short, but rewarding exercise, when you find your character's most obvious traits, to stretch them out so that they become flaws. In fact, the complexity of characters whose best traits are also their worst flaws is what makes them (and those usually good and admirable traits,) so popular anyway. It all depends on how people outside of your cone of author-endorsement of the character's actions would really perceive the trait. If it helps, start thinking of people and characters who annoy you a little and work backwards, identifying what annoys you about them and if there is any good to be had from that trait, or what defining character trait it stems from.
Avoid Contradiction: This falls under the "show, don't tell," rule. People are going to believe nothing that the narration says about a character if their own actions, words and internal monologues don't match up with it. For example, you can describe a character as "stoic" all you like, but if they break down crying in the first few pages, or their internal monologue whines on about minor inconveniences, people are going to put down the book in disgust. (Pro tip; it's easier to write stoic characters in third person limited, so after 30 chapters if they shed a manly tear people suddenly realize "my god, he's been suffering in silence this whole time!" First person tends to mention whatever their trauma is far too soon for it to be effective.) You can claim that a character is kind and selfless, but if we only see her worrying about herself and being irritated and pissy with everyone she comes into contact with, we're going to scream liar.
A good way to do this is to allow the plot to change from what you first envisioned. Your brain works a lot while the actual writing takes place, and it tends to re-arrange things to make more sense. Once you've established a characterization with or without conscious decision, don't throw it away in order to stick to your original outline when those actions would make no sense. Your character's personality comes out through their actions and their dialogue, so save your witticisms, your badassness, your super-cute-shrinking-violet behavior, and your snappy one-liners for characters who would actually conceivably act that way. For example, you can count on Loki of Avengers fame for weaseling out of situations and making snappy comments, but having him blush over every little thing and be at a loss for words doesn't fly. (The man faced down the combined wrath of the adventures and calmly accepted a drink before they kicked his ass – a little witty repartee is not something that's going to floor him.)
Bottom line, sacrifice your "that would be cool," and "that's so cute!" moments for the purposes of the plot. If you're not interested in having a consistent plot and consistent characters, don't be surprised when people complain about the lack of said plot.
Assign Priorities: Because when your heroes are out ass-kicking, they shouldn't be worried about how teammate X will ask them to prom. Seriously. Once you've established the interaction and relationship between a pair of characters, most characters should know each other well enough, especially if they're save-the-world-buddies, that they'll be somewhat confident that the other person will think well of them and be there for them.
Plus, you know, prom won't happen if the world ends, so worrying about the end of the world is a little more important. Ditto for anything that actually threatens the characters' lives, health, and well-being: it gets priority over stuff that can be put off or doesn't threaten them that much. This is why so many series become insufferable when a romantic plot line takes over the progression of events – because the characters are focusing on things that the audience can tell are of only secondary importance, and because they'd be a hell of a lot more effective if they actually focused for a couple days. A healthy romance can survive one partner having to take care of some business alone for a little while.
On the other hand, one of the most surefire ways to establish a character as a little messed-up is to give them unrealistic priorities. For example, a villain who put killing off members of his opposition before obtaining useful information from them is probably consumed by hatred for them – and that lack of information, and tendency to kill before he asks questions, makes a nicely consistent vehicle for his demise. He won't have the necessary information to hold off his enemies, because he hasn't bothered to obtain it.
In a tragic example, imagine a teenager who works very hard on their grades and violin lessons because succeeding in those things is the only thing that gains them approval from their parents. This teen might neglect other healthy or even important things (such as friendships, their responsibilities outside of school, or even contributing to helping save the world with your other teen protagonists,) in order to secure the approval that those activities provide. They may neglect their mental and physical health to get what they want.
Knowing your characters' personalities as well as how they were raised and how they interact with other people is the key to setting priorities that are realistic for them. For example, in the Chronicles of Narnia, it would be unrealistic to have had Edmund seeking his father's approval, because his father is out of the picture throughout the book. Wanting to be seen as grown up and responsible like Peter is a much more realistic goal for his age and his relationships to the rest of the family, especially when combined with the wish to be favored by his siblings like Lucy, the baby of the family. And of course, given that Peter and Susan have taken on parental roles for Lucy, which Edmund rejects because he sees them as bossy, the appeal of being the most important and deciding his own future is one that the White Witch knows he can't resist, at least initially.
Be prepared to work hard on keeping stuff consistent: The more stuff you bring in about a character, the more we expect it all to make some sort of sense, or at least be memorable. For example, I couldn't tell you Harry Dresden's favorite food, though I've read six of the many, many books that Jim Butcher has written about him. Why? Well, it's never been important. However, Harry's loose-cannon attitude and his propensity for killing things with fire has made more and more sense as the series has gone on, and so does the way which the other wizards in the universe treat him. Harry even acknowledges that he wouldn't trust himself in their position, given the context of his actions throughout the series so far and the circumstances surrounding his training as a wizard. [Spoiler: as of where I am in the series – I finished Proven Guilty before the school year started - Harry has not only killed (in self-defense, though most don't know that) his first wizzarding mentor, weaseled his way out of being executed for it, started a war between the seriously under-strength wizards council and the vampires, taken down a massive werewolf, and basically thumbed his nose at the council repeatedly with a combination of burning stuff up and truly terrible latin, but somehow managed to get himself appointed as a warden to said council, stop the destruction of the world (or at least something approaching Armageddon) twice, and taken on a dangerous rogue wizard as an apprentice. No one is sure whether to thank him or run away screaming.]
If you skipped the spoiler, let me summarize: the more shit we're expected to believe happened in a character's life, the better organized it has to be. A couple of lucky coincidences are fine for a character that gets very little done under their own power, or whose plots are less than earth-shaking. Throw too many in for a character who has a reputation of being a badass and we start to think that they aren't a badass at all, just being unjustly favored by the author.
Also, when you have plots that are about saving the world, fighting inner demons, and blowing stuff up, it really breaks the mood to put in the fact that your character really likes orange soda. I mean, you can pull it off – say, at the beginning, your tough guy walks into a bar and takes a bit of amusing flak for ordering an orange soda instead of a beer – but just mentioning the fact can't be too important to your plot. It can be good for characterization – say, establishing that this guy is calm in the face of being heckled for drinking orange soda – but overall, it's an extra detail that wouldn't have changed his character if it were removed, and it's another thing that you have to keep track of. If he always drinks orange soda, you're going to have to have him go out with the guys after a tough mission and drink orange soda, not a beer. Done well, this becomes a series in-joke, done badly, this causes readers to drop out of the story and bang their heads against the nearest wall because nothing is happening.
Keep track of your characters' responses to different situations. Be certain that you can tell if another character would have reacted the same in the exact same situation. This will make sure that your readers remember who is who and that they will root for somebody. Soon enough, you'll be able to do it without quite as much conscious effort.
So, I'm mostly back from NaNo and papers induced hiatus. I'll be updating on something more like a regular basis now, no guarantees as to what. To those of you who are interested, November sucked eggs, and I'm done with papers at least until the New Year now.
Special thanks to Lolitroy for kicking my ass into gear and reminding me that I had something lined up for this chapter.