Say it out loud. If the dialogue sounds weird, fix it. This one pretty much says it all right there. One of the biggest problems with writing dialogue is making it sound natural; reading it out loud helps immensely with that. Yes, you will probably look like you're insane if there are any non-writers around when you do it, but your story will sound better.
Feel free to ignore the rules of grammar.
Young!Gus: And they have to have a love of correct grammar.
Young!Shawn: That's not a rule!
Young!Gus: You said we could have one special rule. That's mine.
Young!Shawn: And that's the best rule you could think of?
Young!Gus: I think you mean that's the best rule of which I could think.
Young!Shawn: [Stands up] I'm not being in a club with this!
Young!Gus: Fine! I don't need you and your misplaced prepositions!
Psych 2.14 - Dis-Lodged
I chose the above bit of dialogue because it was the most fitting example I could think of for this. While the sentence, "That's the best rule of which you could think?" is grammatically correct, it just doesn't sound natural. Unless your character is a stuffy, uptight and/or very proper they aren't going to be saying things like that.
Yes, there are rules for grammar that you should follow for writing but a large portion of those rules can, and should, be ignored while writing dialogue. Because people don't follow them, especially when angry. Sample angry sentences that I have said or heard spoken include, "Don't none of you wanna even start with me right now," and, "I will fuck your shit right the hell up."
Remember your characters. There are some phrases and ways of speaking that will not work for certain characters. Jared and Jensen, despite being from Texas, cannot get away with starting a sentence with, "Bless her heart," because they are, in fact, not women. And none of the boys will say something like, "You've the right of way," because that is a U.K. speech pattern. The proper American sentence would be, "You have the right of way," or, "You've got the right of way," or, even, "You got the right of way." More on this later.
Remember who your characters are. While Jared would, and has, said, "I'm sweatin' like a whore in church," this is not something Sam Winchester would say. Much in the same way Dean probably won't be saying, "Y'all" any time soon unless it's part of a disguise. I highly suggest watching an episode, or at least a clip featuring your character, before writing their voice if you feel unsure of it.
You get a fair amount of leeway in RPS because a good deal of what we hear them say (I include magazine interviews when I say this) is scripted, but you're still expected to adhere to a few main things.
Seriously. Speak it out loud until it sounds right. I cannot stress this enough. It might sound good when you write it, but if you say it and you can't get the pacing right then you need to work on it more. I will routinely spend ten or fifteen minutes on one line of dialogue or one exchange because it doesn't feel right to me. Given, I'm OCD (and not always that fun kind you have on the net), and thus tend to overdo everything, but spending time on your dialogue is still important. Don't just skip over it just because you'd like to get back to the narration. It takes time to get it right but it's worth it.
Accents. Yes, Jared and Jensen are from Texas. And, yes, Sam and Dean travel all around the country. But (and I say this as a Gambit fan from X-Men fandom) if you get the urge to write their accents, you need to take a gigantic step back and find someone to talk you out of it. Yes, writing accents can be good, since it helps the reader hear the words in their head, but it is far too easy to go overboard with the apostrophes and missing letters.
A sentence here or there that resembles, "I'm'a run down to the lake," or, "Don't go flyin' off at the handle," are okay to a point.
Do not – I repeat – do not try and spell out anything phonetically. Some of you may be laughing, but others are cringing and remembering when they used to do it. The only excuse for dialogue that is incomprehensible is if it is meant to be, in which case I would suggest telling the person has a thick accent that your character(s) can't understand rather than showing us through what looks like random mashes on the keyboard.
Know when to appreciate the silence. Don't use talking just for the sake of words; if the scene you are writing can work just as well without dialogue, then don't force it in. If the dialogue doesn't fit, it will show and the fic will be weakened because of it.
Stereotypes exist for a reason.
Boys are much more like the Jackass/CKY guys than any of you know; those guys are just the group of friends who were smart enough to videotape it and lucky enough to get a lot of attention. If two boys are in a room and one takes a shower, he knows to expect the toilet to get flushed at least once, why? Because this is what boys do.
Boys get drunk and pay each other to do things (eat a dog biscuit, lick a dog's ass, etc. Sadly, these are examples pulled from an actual night with my brothers and one of my Dad's friends). They get bored and beat each other black and blue. They fart and then ask if anyone else smells barbecue just to get others to inhale.
This is not be telling you that in order to write Jared and Jensen correctly one of them has to hot-box the other or anything like that; this is just to help you understand their brains. Random-ass questions along the lines of, "If you had to fuck another guy, who would it be?" and, "Who would you go gay for?" are not as completely unheard of as you would think.
"No chick flick moments" means no chick flick moments. Sam and Dean aren't going to be saying, "I love you"—in a brotherly context—out loud anytime soon because that's not what guys do. Brothers say, "I love you" by not killing each other despite the intense want to do so. Every time Sam wakes up with Dean's socks shoved in his mouth and then doesn't maim Dean horribly he's saying, "I love you." Every time Dean doesn't punch Sam in the face for a stupid comment he's saying, "I love you."
(Now, the argument can be made for Sam and Dean saying it in a big gay incestuous context, but that's really a matter of opinion. My personal opinion is that it would be extremely OOC for them but I have seen people argue the other way.)
Boys, and brothers, fight. And when they do they aim to hurt because boys fight dirty. There's this thing called "dead arm," where you make a fist and then sock someone repeatedly in the exact same place until their arm starts to go dumb. "Dead leg" is basically the exact same thing, but to your thigh. Boys don't have to be mad at each other to get into a physical fight; many times it's just a matter of a couple of guys just wailing on each other until someone wusses out and cries, "Uncle!"
They also forgive easily and without apologizing.
Boys think pain is fun (and funny). I recently saw someone complain about Sam's light-heartedness with Dean's wounds after Mystery Spot. I'm not going to talk about the different types of wounds they've had and the severity, because that's a different meta. What I'd like you all to do is think back to the pilot. Sam gets a hand shoved into his chest by a ghost who is about to try and kill him and then he drives a car through a house. What Dean does next is a boy thing. He smacks Sam on the chest, right over the holes in his shirt that designate where the hand was lunged into his chest. This is what boys do; it's why CKY and Jackass and The Three Stooges are so popular.
I'm not saying that Sam and Dean never worry about each other's injuries; Hunted, All Hell Breaks Loose (part 2) and Mystery Spot are all perfect examples of the boys worrying over each other. But something as small—to them—as a gunshot wound in the shoulder does not merit that kind of worry, only teasing and random poking of the stitches.
Brothers insult each other. They do it often, constantly, and say every non-politically correct thing they can think of when they do it. This isn't necessarily them being mad at each other; this is just boys acting like boys. This section is particularly short because it brings me right into my next one.
Boys are not politically correct when there are no girls around. You cannot write realistic boys without being attacked and/or begging for wank. Remember the wank that the, "Dude, could you be more gay?" line caused. And, you know, that is a guy, that's how they talk. And all the stuff about Sam being a little girl? That's how brothers talk to each other.
My brother, in particular, is possibly the least homophobic person ever and he talks like that. Guys call each other "cocksucker" and use it in a derogatory way. They do the same with the word "gay" and whether or not you would use it personally doesn't matter. My younger brother says it and my older brother said it and so has every guy I've hung around with except for my roommate. And my roommate doesn't not say it because he's gay—he doesn't say it because he actually says, "Gosh!" and other grandmotherly things like that.
It's how Sam and Dean talk. Ideally I would say that people need to stop being so PC but that isn't going to happen. What you need to remember is that these are boys; this is how they talk. Compromise. You don't need to have Dean calling a bad movie "gay" but you need to keep in mind that he doesn't have a very good working brain-to-mouth filter. Same goes for Jared and Jensen, though to a slightly lesser extent. Only slightly though, because we have fan confirmation that Jared curses like a fucking sailor.
(I would like to say, for the record, that when I say, "When there are no girls around," I do not mean that women are all prim and proper and boys are all stupid. I have that in there mostly to stave off the comments that will pop up from people who want to let me know that their brother/best friend/boyfriend/husband think Jackass is stupid and would never call anything "gay" unless they meant homosexual and they meant it in a nice way. And, really, it's the guys who tend to edit themselves around the wimmin-folk.
I mean, we all know we're dirty as hell but there's a reason that women on TV don't like sex and act like prudes. Because guys don't know how chicks are with chicks. And a lot of chicks like to delude themselves about guys.)
Make it sound American:
The dos and do nots. Mind your contractions; they're a dead give-away that the author's primary language is not American English. Instead of, "Dean does not know why Sam has got such a bad attitude; he is the one who wanted to come to the store," do, "Dean doesn't know why Sam's got such a bad attitude; he's the one who wanted to come to the store."
Boots: things that go on your feet (not parts of a car). The Queen's English is not the same as American English. We have mostly the same words, but they don't always mean the same thing.
Dean does not stand in a queue at the gas station; he stands in a line. Sam does not get a gun from the boot; he gets it from the trunk. Jared and Jensen do not live in flats; they live in apartments (or hotels or motels or houses). And none of them use mobiles; they use cell phones, or "cells" as they're more commonly called here. These are, by no means, the only words with different meanings. There are many sites, such as this one, that help UK authors find the correct US word.
See also: List of British words not widely used in the United States, List of words having different meanings in British and American English and The English-to-American Dictionary.
I would like to point out, and stress, that this is not me, the big important American, telling all you foreigners how to act. This is me, the random American reader/writer, telling you what peeves me and a lot of the people I talk to and giving you suggestions as to how to fix them.
Lets's have a go then, oi? UK phrasing, with or without English words, is awkward. What's natural to you isn't to us (and what's natural to us isn't to you). Whereas you would say, "I've not found the correct grave," that's not how Sam or Dean would say it. The American way to say that sentence is, "I haven't found the right grave."
"You guys," "You'se guys," and, "Y'all." Don't forget your dialects. Sam and Dean grew up traveling, so they have no real specified accents. They're fairly malleable. However, depending on where you set your fic, your background characters will be talking differently. What someone in one state calls soda another calls pop and another calls Coke. There are parts of the country where you can walk into a diner, order a Coke, and be asked what kind you want. This doesn't mean Vanilla Coke or Cherry Coke; this means Sprite or Pepsi or whatever. Other examples include the different words for a sandwich: hoagie, sub, grinder, hero, etc.
The background characters in your New York fic will not be saying, "Y'all should get goin'," if they're native New Yorkers, just the same as your native Alabama characters will not be saying, "You'se guys." It just looks messy when you don't do your research.
This part is even for the Americans. If you are writing about a place you've never been to, you should find someone who lives/lived there and have them check through your fic for you for glaring errors. Making up a street or two is okay but when you place a fic in Phoenix, Arizona and make it 90 (which is twenty-thirty degrees lower than the "normal" summer temp here) in the middle of summer or place it in Washington state and make it bright and sunny for a long period of time, there better be a supernatural reason for things as ridiculously unusual for that happening.
Ameri-picking. Don't hesitate to find someone to Ameri-pick your fic for you. I highly recommend getting a beta who has the sole purpose of making sure your fic sounds American. America is such a large, integral part of the show and it carries over into fic. I can't speak for all American writers, but I know what when I read an otherwise good fic that suddenly has Dean yelling out, "Bollocks!" or talking about someone's arse, it throws me right out of the story.
IF YOU ARE WRITING ANYTHING ABOUT OR IN TEXAS FIND SOMEONE FROM THERE AND MAKE THEM LOOK IT OVER OR YOU WILL BE SHISH-KA-BOBBED PAINFULLY. I'm not kidding, they bring "state pride" to a whole new level. If you're writing a fic anywhere specific I would recommend finding someone from there to look it over but especially if you're writing anything that has to do with Texas. I cannot tell you the number of times I have had a Texan friend rant at me because someone had put Jensen on a horse ranch in Richardson or something else like that.
A long drive to you is not a long drive to a Texan. Texas is a big state; Jared and/or Jensen will not be complaining about having to drive an entire hour to get somewhere. Seriously, watch the comments, I bet you a Texan or two is going to pipe in and talk about this. I've sat on the phone with Texas friends in the middle of the night before because they were coming home from the weekend and were an hour and a half into their three hour drive.
When in doubt; ask. If you're unsure about the accuracy of some Texas plot point—or any plot point—ask before using it. Don't just guess or assume no one will notice, because they will.
Stop with the repeating of your repetition already! If you say something once in fic you've said it a thousand times; there is little-to-no need to repeating yourself. You don't need to remind your readers every thousand words that Jared is mesmerized by Jensen's eyes, we will remember. If you keep mentioning something in particular over and over again you will annoy and/or insult your reader.
You only have to mention once that Jensen has a vintage Thunderbird he's been rebuilding since he was eight; if you keep repeating this you are implying that the reader isn't smart enough to remember this fact. Also, you are implying that this is something incredibly vital to the plot, and if you mention the Thunderbird several times and then it doesn't end up totaled or vital then you will just piss your reader off.
"Clex, why do you keep repeating my name, Clex?" They know each other's names; stop making them address each other formally. Yes, in the show Sam and Dean will constantly use each other's names when talking to each other—it doesn't mean you should do it in fic. Television, while entertaining, does not paint a decent picture of reality. If someone is looking for another person, they can call out their name. Same when talking about them or, say, in a dialogue-only conversation (I wouldn't recommend it but that's just my personal opinion). Unless a point is being stressed, such as, "Dean, I told you to stop that," or, "Come on, Sammy, have a little fun," there's not a need to have two people talking exclusively to each other call each other by name. This goes back to the part where you need to read your dialogue out loud.
Yes, this goes for AU fic too. If your characters are just meeting for the first time they may use each other's names a little excessively with the excuse of trying to remember the names, but after a paragraph or two you need to assume that they remember each other's names by now.
In conclusion; people don't speak right when they talk, boys poke each other in their bruises, Jared and Jensen are boys (along with Sam and Dean), Americans finally get their revenge on Harry Potter fandom, and Texans will cut a bitch if you scorn them.