No, not the so-called authorial Mary Sue/Marty Stu school of fiction, where the reader realizes that the protagonist is the author, just perfected in every way. I can think of one, maybe two people who got away with that for one book, possibly two. Stan Lee slipping himself into the comics on occasion doesn’t count, because that was deliberately breaking the Fourth Wall. (Ditto the movies. Although sometimes playing “Is that the author?” can be entertaining.)
No, I was thinking about how we as story tellers put ourselves unwittingly into stories, and what that means for editing and story tone. I know of one author who slid into a rut that might be personal experience, or it might be ideology, or yes. As a reader, I’ve backed away from that series for a while, for that and other reasons.
Accidentally slipping too much Author into the story might not happen to other people. I started writing as a way to vent, back *coughcough* years ago. I got over that, or so I thought, then started again when I couldn’t tell a senior and indispensable coworker to go to perdition and take a certain airplane with him. It flared up again in grad school. Occasionally I still write a fight scene or the like as a form of stress release. I’m not the only one among the writers that I know. Society* has no problem with writers who bump-off characters, especially after the character bounces up and down demanding, “Give it to me, give it to me!”
The difficulty came when author, narrative voice, and character blended together. I couldn’t see it, although I knew that something about the story felt odd compared to other books. It has happened in the past, and I recognized it then because it was not only obvious but it verged on bathos. The solution was to go through and look at descriptions and word choices, so that character thoughts and observations and omniscient observer thoughts and observations were clearly different. “Would the character know this? If so, how?” Once I caught the problem them, it was relatively painless to go through and fix any confusing passages and details.
This time, I couldn’t see it. Part of the problem stemmed from not sitting down and working out some key details for the character’s motivations and developmental arc. If this were a Jack Reacher-type or Paladin-type figure, where character development is not expected by readers, it would be OK, or even good. However, the series requires growth and development, shifts in motivations and in some ways growing up again. Think of an immigrant to the US back in the 1870s-1880s who is out of reach of her basic culture. The village support network is gone. The village gossip and limits on what she can do are also gone. (Like the great grandmother of one of MomRed’s coworkers, who fled her village on Sicily in the 1890s when informed that at age 14 she would be married to a 60+ year old Mafia-boss type. The young woman ran away and ended up in Grand Island, Nebraska working in a boarding house. Culture shift!) I’m not that fish in new waters, now. I once was, albeit temporarily. Too much of that period and some other things crept into the story and weakened it. I didn’t see it until a very observant beta reader said, “You know, something’s off kilter here” and that led to both of us seeing the problem, and me fixing a lot of it.
Am I saying that you have to keep your prose pristine and uncontaminated by personal experience and feelings? Oh heck no! What I am saying is that sometimes we as authors need to step back and think about why a character or story feels off. Was/is something going on that leaks, and not in an obvious way? The answer could be “No, that’s just how this story works, and it’s going to be Strange.” Other times, some changes might be called for to improve the story and foreshadow changes. Making sure that the language matches the character might be part of that, as it was in this case.
*And then there’s the fan who is too invested in a character. Stephen King’s Misery. Enough said.
Mad Genius Club 
Aesthetic distance is important
Sometimes you can learn to fix it; sometimes you just learn what to avoid. For example… Thrillers typically have a Villain™ (one or many) driving the plot, the motivations of whom I always struggle with caring about. In the conventions of that genre, they’re a given, and so the weakness is simply shrugged off when I read.
They’re not uncommon in Epic Fantasy, either, but I’ve learned that my own best stories are not based on villains doing the primary driving. I’m much better at including a mix of situations that the hero has to deal with that are more similar to real life and less deliberate-villain-driven. Sure, he has antagonists, but they have their own problems that he occasionally intersects with, almost by accident — they don’t drive the main plots. The inconvenient real/economic/chaotic world provides plenty of interference all on its own. That works much better for me.
Yeah, sure, it would be good to become a master of any narrative/character situation, but life is short, I started writing late, and I’m not really worried that there’s gonna be a test. I have lots of stories to tell, and who really cares how varied the forms are?
Yeah, this is kind of my attitude. I write primarily to entertain myself. I do generally try to tell the story to the best of my abilities and make it accessible to people who aren’t coming at it from my weird POV. If it works, great; if not, better luck next time.