Here's an article on stories with multiple personality disorder.
That's not entirely what this is about. Watching several of the new genre made-for-tv series on Netflix and Hulu, I frequently stumble across something I've had to work on in my own writing: Would this character really do this?
The most recent example on tv was from Agents of Shield where the character of Quake goes into a battle with the Big Bad of the current story arc and doesn't smoosh him into the wall, nor once the Big Bad is down, does she take him with her to be put into one of the quarantine cubes at Shield. Quake was trained to fight specifically by Mae, the badass warrior woman who takes no prisoners. She was also trained as an Inhuman to work with her ability. She has also been trained in "what you do with a Big Bad" by Shield itself. In this fight, she tosses him back from her and then goes in to fight hand to hand. Mae would have never trained her to do this. She eventually beats him back and leaves him (so the writer's can have him improved like the bionic man and return to haunt them). Shield would have never trained her to do that. She would have crushed him full stop as her first move and then she would have taken him back to the jet. The writer let go of the "rules" of that show's universe to allow the trope of the Big Bad escaping to plague them once again to occur.
Writers need to stand back and look at the motivations of their characters and ask themselves:
Why? Why are they doing this - Why is this stay at home mom... why is this super villain... why is this charming man about town... putting up with... setting up... ignoring... this Rube Goldburgian machination to make this other thing happen? Is there is no clear motivation for action other than there wouldn't be a story without it? Then you have a serious plot hole that needs to be filled in. Also, characters change as a writer writes, and it just may be that the character you wind up with may not have ever been motivated to commit those earlier actions.
If this goal is really what the character wants - is that really how they would really go about accomplishing it? Does she really set up a convoluted scheme with a local butcher to get an usual cut of meat to her local favorite restaurant and ask them to prepare it in the unique way, using these rare far eastern ingredients she had to special order in to impress her teacher that she was more worldly than she seemed ... or ... does she weigh the pros and cons of just talking to her teacher and fessing up?
What are the other characters doing? Are they just getting out of the main characters way as they do things, or do they have something to say? The main character is an FBI agent on a tough case, but he's acting like a lone wolf and is for some unknown motivation not using the FBI resources. Does no one stop him and remind him of protocol? Does no one say, "Um... you have actual assigned cases that we work on as a group. Your job's in jeopardy by working on this side case instead of doing your assigned cases..." or "Um... you're part of a huge unit of investigators and we work together... and we have all these computers that we use to figure out the stuff you're trying to poorly discover on your own..." Sometimes figuring out motivation can undo an entire story idea. Sometimes it just means from a certain plot point forward - it's not the book you were planning to write, but a new story that has yet to be written.
As a writer you have to be able to step back from a work, even let that work sit in a drawer for a period time, until you can come back to it with fresh eyes and ask yourself, is this really what would happen. Is it too clever or too cliche for its own good? Did you like writing that twist ending so much that you are overlooking the fact that it makes no sense to the story? I've been very sad to let pages upon pages of plot drift away, going up in smoke because the actions of the characters made no sense. It allowed the neat little bow to go right on top of the finished product, and it was sure fun to write, but once pulled apart and reworked, the story only became stronger because the characters acted honestly and the plot will go into new directions that are better than what was there before. Sometimes you're not so lucky and the whole story goes away, but better to let it go than to add another cliche filled book to the heap of genre cliches already out there.