I don't want to be mean. Actually, I've caught Mary Sues lurking in some of my own stories. And, I'm sorry to say, Mary Sues have even found their way into a lot of popular books and movies (I'm not going to start listing names because I'll just make people made at me).
But a Mary Sue is the result of laziness. She's there to be awesome so that the author doesn't have to be. Instead of taking the time to create a true-to-life character with real problems and real needs that readers can relate to and grow with, the author simply whips up a Swiss army pocketknife who will amaze readers with all her incredible abilities. It's easier, but not better.
>>By the way, my acknowledgements to Blake Snyder for some of these fixes. If you haven't read his book Save the Cat, you really should; and yes, this is an unabashed advertisement. He's one of the best story-crafting gurus out there and I tout his ideas constantly--but guess what--they WORK. Okay, done with the ads.<<
She's drop dead gorgeous This is a big indicator of a Mary Sue--she's either really pretty, or (if it's a guy) really handsome. Some authors catch this mistake and go too far in the opposite direction by making the character fat or ugly. That doesn't necessarily keep her from being a Mary Sue but it does help a little.
Fix: I personally prefer to describe a character--particularly the main character--as little as possible. You only need to give the reader a few details and you don't have to tell the reader whether she's pretty or not. Readers will figure that out soon enough from the way other people treat her or the way she views herself.
She has awesome abilities It's fine to give your character some special skills but frankly that sort of thing is really, really overdone. It doesn't impress me anymore. In fact if you want to impress anyone with it you have to give the Mary Sue a skill that's either really spectacular or really bizarre.
Fix: You could simply not give the character amazing skills (gasp). Or, if you really want her to have great abilities, don't just hand them to her. Make her have to work hard to learn the skills. Instead of being born with them she must acquire them through severe trauma. Don't make it easy. Also, consider making the skill something that comes with drawbacks (like Elsa in Frozen has an ice power but can't control it).
Everyone likes her If the Mary Sue is a girl, usually several guys in the story fall in love with her. She's generally universally popular or at least universally respected. Even if you try to avoid this mistake, you can swing too far in the other direction by making everyone in the story hate her. It's still the same problem--she's the centre of attention and all the other characters are preoccupied with what's going to happen to her. Seriously, in real life hardly anyone cares what you're going through or what happens to you, and those who do are too busy with their own problems to care very much. So why should it be different for your character?
Fix: Keep in mind that although the story is about this character, the other characters don't know that. As far as they're concerned she's unimportant. --Even if she's the chosen one the prophecy speaks of. Refrain from making other characters constantly give your hero pep talks or endlessly discuss him with each other.
She never fails Nobody likes to fail and not many people enjoy seeing characters they like fail. But guess what: if your character fails--if her plans go bust or she screws up her big moment--it makes the readers care ten times more about her. Because we all know what it's like to fail. And we all want to know that a failure can go on to be a success.
Fix: Make her screw up and--this is important--make it HER fault. Don't make her just clean up after everyone else's mistakes. Make her fail in a painful, personal way.
She has no need of change Every story is about a journey (physical, emotional, spiritual, or otherwise). The main character needs to change during the story, but Mary Sue is already awesome--she doesn't have to make the journey to get there.
Fix: Here's one of Blake Snyder's tips: Take her a step back. She's not quite awesome yet. Maybe she's insecure. Maybe she's overly confident. Maybe she doesn't care enough about other people. Rewind her a little bit and let us watch her find what she's missing. Better yet, take her all the way back to where she started and let us see her whole journey.
The story isn't about her I saved this one for last because honestly, this problem has given me the most angst of them all. I write a whole lovely story only to find that the real hero of the story is the best friend, boyfriend, or some random, minor character who popped into the story for two scenes.
Fix: When this happens, you have two options: change who the hero is or change the story. To locate the true hero/heroine ask yourself two questions. "What is the journey this story is about?" and "Who needs to make this journey?" Disney/Pixar's movie "Up" is a story about how relationships are more important than dreams. Who needed to make that journey? --Mr. Frederickson, a lonely old man whose childhood dream was never realised. It might have been tempting to make the movie about the boy Russell, since the movie was for kids, but doing so would have made the movie's message less powerful.
So there you have it! Six symptoms of a Mary Sue and how to cure them. I am seriously tempted to do a post about Mary Sues in popular books and movies, but I'd make too many people mad and no one would read my blog. :(