I warn you, I'm about to rant.
I take no prisoners when I rant. I'm going to TRY and avoid naming names, but I can't make any promises.
So you know what I hate? Original characters that have no idea of their frakking place.
I've read a few stories where Sarah chose to leave Chuck because her feelings were compromised and she didn't want to hurt him, things went pear-shaped, and now it's a couple of years later, Sarah is mysteriously in trouble because she's evidently just a bad agent (which is another rant for another day, and it will take no prisoners, yell at the Chuck writing staff, and leave no cliche unmurdered), and Chuck comes into her life as this total badass who can do everything, saving the day without a drop of emotion. He usually has a new female partner, somebody very good at her job, young, usually brunette, and outspoken.
And what happens? That partner starts ranting on and on, blaming Sarah for everything up to and including Chernobyl. She rakes Sarah over the coals for a good half hour or more, while Sarah is so overcome with guilt that she just...takes it.
Maybe it's my feminist roots coming out here, and I don't care, but let me change that "no" to "no way in freaking hell." I really hate it when fics do this, when they introduce these original characters just to make Sarah feel guilty over her actions and to provide exposition because they feel it makes the story more interesting. It's lazy writing, and it's freakishly insulting to a character that I know and love. Every time I see this in fic, I just want to unleash Fates Sarah on the OC's ass and watch the carnage unfold. No way in hell would Fates Sarah take that lying down. She would cut a bitch. Well, actually, she would give the bitch the ice treatment and try to figure out what the hell is up with Chuck, and if this OC character that the writer has not given me as a reader any reason for emotional investment got in her way, she would drop a bitch.
Okay, Fates Sarah...probably not the best example for this, as she's one of the angrier Sarahs in the fandom, but still.
If you're going to have an OC yell at a main character, fine. That happens in real life, but don't expect me to buy characters who are strong, opinionated, and good at their jobs not telling this complete stranger to butt the hell out. Don't you DARE try to tell me that the spine I have admired on a character for two seasons (ignoring S3 Sarah because that's not a person, that's a plot chameleon) is just magically gone because you have this OC that I don't care about at all and you think that OC has a lot of good things to say. Let them say those things, but let them face the consequences, too, which should be, "Mind your own #$%@ing business, bitch."
I'm not saying that the OC won't be saying things Sarah needs to hear (though it's rarely the case), it's just that if it's something to do with Chuck, that should be between Chuck and Sarah and Sarah has the right and the responsibility to tell anybody who tries to tell her what's up to "shut the #$%@ up." A great example of a fic that does this well is Double Agent where Sarah repeatedly has problems with Carina, who is pushy and protective of Chuck, and she's just like, "Whatever, bitch." A lot of the time, Carina's right, and it's just really well-written conflict.
It should probably worry me that I'm using an emotionally-damaged-beyond-repair Sarah as opposed to just an emotionally damaged Sarah as an example of how Sarah should act all the time, but I'm 100% okay with that.
And for the love of little Intersects, if you're going to have an OC that needs to tell one of my main characters what-for, put some work into that character. Make me see why that character is right, and why that character has the right to be able to talk to any of the mains I love in that way. Otherwise, quit using a description and a wig as a mouthpiece. You're mad at the character for doing something you made them do. I get it. But don't expect me to buy it. If you're going to have an OC, round that character out. If not, you don't need that OC.
And that is my rant. It's actually a much nicer one than the one about how I think Armadilloi is Stephenie Meyer in disguise.