A “Mary Sue” is a character who is too good to be true. They have all the skills, they save the day, they sacrifice themselves to be the hero everyone remembers. In fiction this kind of character is derided as being the overly perfect man or woman who has the all best qualities and none of the flaws.
I think the general attitude towards this kind of character is a little misconceived. There’s always been the character who was good at everything, the hero, the beauty, the chosen one. Whether it’s James Bond or Cinderella or Neo. And in some genres, like Romance or Spy Thrillers, this kind of character is pretty much standard.
The thing that stands out with “Mary Sue” types I come across isn’t what they looks like, or what abilities they have, it’s what happens to them. And it’s what they do. Or don’t do.
She doesn’t do anything. She waits for things to happen to her. She never asked to be the centre of attention, she just is. She tends to be blameless, without guile or ambition, yet fate decides to land her in the middle of everything. Somehow all the attention is focused on her.
The good looking boys are drawn to the girl who keeps to herself.
The bullied boy is the true heir to the throne.
The wish-fulfilment fantasy isn’t that Mary Sue is the best looking, most talented character, that’s too much like boasting (and more importantly very unlikely to come true). In a culture where people want fame and fortune to drop into their laps with the least amount of effort possible, Mary Sue is the character who becomes the most important person in the world without even trying.
If your main character stumbles into the middle of a plot in which they are the point of reference for everyone else for no particular reason, you may have a Mary Sue.
Why does she draw all the focus from the other characters? What has she done that makes her so crucial to every single other character in the book?
If your character doesn’t have a goal in mind, if they’re all reaction and no action, if attraction is based on intangible qualities that cannot be named, you may have a Mary Sue.
Of course, if the MC has proactively done stuff that has brought the Eye of Sauron upon her, if she has a plan to get what she wants and the fallout has got people talking about her, then she is no Mary Sue. Active characters who drive the story forward are too busy to slump into a self-pitying heap until the local male-model/billionaire who has a thing for average looking brunettes happens to walk by.
A writer can snap his fingers and make every other character in the story be enthralled by Mary Sue’s every move, but he can’t do that with the reader.
Of course we would all like things to turn out like that in our real lives, the happy ending daydream fantasy we all have. But a story about chance encounters and contrived good fortune doesn’t make a character any more interesting than they were sat at home wondering what to do with themselves.
Mary Sue doesn’t earn what she gets. She doesn’t work for it and when things get tough, she gets a helping hand: the coincidence, the lucky turn of events, the convenient assistance at just the right moment. Avoid Mary Sue by giving your MC a goal in life. If the events in your story had never happened, what would your MC be doing? Don’t have life stop and start with the book, make your plot interrupt a life already being lived.