advice: how to tell if your character is a mary sue
If every single poor decision your character makes falls under one of these categories, you are probably writing a Sue. Don't worry, I've included tips on how to fix it.
Now I want to start off by saying that none of these qualities are, on their own, indicators of a Sue. For example, it's totally plausible that a well-rounded character would make bad decisions when drunk. However, for your character to be believable, they've got to be able to make bad decisions while sober, too.
1. The character has to be drunk or high in order to make a bad decision. Bonus points if it's in your bio that character rarely or never drinks or uses.
Why it's a Sue thing:
A really, really common trait in Sues is that they can't make mistakes for themselves- they have to be tricked or forced into it. Flaws are the things that lead us towards making mistakes in the first place. While it's perfectly fine for characters to make mistakes while drunk or high (goodness knows it happens in real life), that can't be the only time they make mistakes- it's lazy characterization.
How to un-Sue it:
If you find yourself leaning too heavily on this trope, get rid of the alcohol/drugs. Are you getting your character drunk to get them into a fight? Ask yourself what it would take to get the same reaction while sober. Are you getting them high so they'll make a confession? If you really think about it, you can probably figure out how to get them to confess while sober. Really delve into your character's mind and figure out what it takes to break them and make them say or do things that they wouldn't normally say or do.
2. The character makes out-of-character mistakes.
For example:
The character isn't normally a snoop, but walks in on something they shouldn't see.
The character isn't normally clumsy but inexplicably falls/breaks something important.
The character is normally responsible but inexplicably forgets/loses something important.
Why it's a Sue thing:
It shows a lack of consistent characterization. What's the point of developing a character if major plot points are going to hinge on them acting out-of-character for no reason?
How to un-Sue it:
Integrate the flaw into their personality. Commit to making your character an invasive snoop or an inattentive klutz (note how I said inattentive! Keep in mind that clumsiness on its own is not a flaw) or carelessly forgetful. Next time something interesting happens, make them go nosing around where they don't belong and plot for realistic consequences to come of it. Make them the kind of person who is so lost in their own little world that they don't pay attention to what's going on around them. Make them a selfish character who only remembers things that are important to themselves.
Figure out a different, more in-character way to get to the mistake. Maybe your character isn't nosy, but maybe they have a competitive streak and can't back down from a challenge, so when someone dares them to look at something off-limits, they can't bring themselves to say no. Maybe your character isn't clumsy, but maybe they are careless and forget that they left the maguffin on top of the car before driving away. Maybe your character isn't irresponsible, but maybe they overestimate their own capabilities and get so bogged down in work they don't have time or energy for that they miss something really big and important. Make these characteristics consistent and make them have real, in-story consequences.
Spend time developing and clarifying the character's motive. Your character's never been nosy before- why is she suddenly nosy now? Give specific, in-story reasons for your characters to do things that wouldn't make sense under normal circumstances. Maybe your character's never snooped before- instead of having them randomly stumble across important information, give them a reason to go looking for that information and then send them after it (i.e. I never stick my nose in other people's business, but I just found out my sibling might be in a secret relationship with their high school teacher- I'm going to snoop in their phone). Let the character feel uncomfortable or out of their element with what they're doing, make them bad at it, make them feel guilty about it. Your character's not clumsy or forgetful- why has that changed all of a sudden? Were they out late the night before? Are they distracted by some other problem in their life? Don't just let stuff happen without reason- make the motive clear so that the story is believable.
4. The character's mistake is the direct result of (or, at least, closely related to) something that's not their fault. For example:
The character is already painfully shy or has a stutter and messes up in a public speech.
The character has a panic attack related to a phobia or some past trauma and is unable to complete an important task.
The character has no training and therefore loses some kind of fight or match.
The character's physical or mental condition gets in the way of completing a task.
The character's overbearing mother embarrasses them at the worst possible moment.
Why it's a Sue thing:
Don't get me wrong- these kinds of things are part of normal, every day life and should be considerations when working on your character. But these are not flaws. When they are treated as flaws, they're lazy shortcuts toward gaining sympathy for your character without making them have to do anything unlikable.
How to un-Sue it:
Realize that they do not count as a character flaws. While your character may see these things as flaws, and the other characters with whom they interact may see these things as flaws, as a typist you need to remember that these things are not flaws on their own. A good rule of thumb is that for something to be a flaw, it has to be- at least to some degree- in the character's control. A character can't choose to have a stutter/phobia/physical or mental condition/their parents. These kinds of things are certainly important to a character, but they're not flaws, they're circumstances. Which means you've got to think of other actual flaws to give your character.
5. The character is "too nice". Overly loyal, too giving, too kind, too friendly... conversely, if the character is "rude but doesn't mean it", too sarcastic but it's funny so no one cares- things that are traits but not really flaws.
Why it's a Sue thing:
This is like when people go on job interviews and say things like "I'm too dedicated to my job" or "I'm a bit of a perfectionist" when asked to name a flaw. These things can be flaws, but unless you actually understand how to make them flaws, they're little more than humblebrags. Same with rudeness or sarcasm- if it's done in a way that it just comes off as funny, it doesn't count as a flaw- if anything, humor is usually considered a positive trait.
How to un-Sue it:
Think through the specific ways these flaws are actually problematic for your character. If they're the only flaws your character has, they're going to have to have an impact. Your character is too nice? Maybe they don't know how to say no or they're afraid to say anything for fear of hurting someone's feelings. Maybe they constantly stick their nose in other people's conflicts where they don't belong and try to solve problems they have no part in. Overly loyal? Maybe they're possessive or controlling. Maybe they have stalkerish tendencies. Too friendly? Maybe they have terrible judgement in people and tend to put trust in people they shouldn't. Rude or sarcastic, but with good intentions? Maybe they're too proud to admit when they're wrong and will tell the other person they're being too sensitive if they get offended. Maybe they just plain cross the line too often.
I want to end by reiterating that none of these things are automatic Sue indicators. But, if you're consistently pulling from these tricks to come up with "flaws" for your character, maybe you need to head back to the drawing board for a bit.