I know that Mary Sue/Gary Stu are fanfic terms, but I've seen many people using these terms to describe characters in other forms of fiction as well. I can easily spot a Mary Sue in fanfiction, but the lines get hazier for me in other forms of fiction. In your opinion, outside of fanfiction, what do you think makes a character a Sue/Stu? Is it the absence of "legitimate" flaws in the midst of "too many" strengths? Is it that their strength is far superior to any other character? What do you think is the best way for authors to avoid this trope? Or do you only use the term when you're talking about fanfic characters?