This comes up because I seem to be writing Boston Legal fic as a Sir Thomas Malory pastiche, and while Late Middle English *grammar* I can manage pretty instinctively, I keep having to stop and think about whether to use 'you' or 'thou'. Okay, so it's not as hard as it *could* be - clearly King Dennys uses 'thou' to *everyone*, because *everyone* is inferior to Dennys of Cranepool. Sir Alain du Côte, by contrast, always uses 'you', because Sir Alain is familiar with nobody - except with Dennys, and occasionally with whatever damosel he's sleeping with. I have a feeling that Sir Jeraint, the Knight of the Hands, uses 'you' with absolutely everyone, except when he's very nervous and forgets, because it seems like that's one of the social things Asperger's would make difficult. It's when I start with the characters who are more or less normal that I run into trouble.
I suppose that this isn't that big of a difficulty - after all, many modern languages still make the distinction, English being one of the first to drop it, so translators must handle the issue. But when I speak Spanish I usually just go with 'usted' for everyone, because none of my teachers bothered to explain how to choose (and since they all spoke different dialects anyway, they'd probably have contradicted each other), and anyway it's one less set of conjugations to remember. I have a feeling I sound comically old-fashioned, but then my grammar sucks so bad they'd be laughing at me anyway, and better too formal than too informal - anyway usted is the direct translation of English 'you'. And in English I only fall naturally into 'thou' when cursing people.
This is actually the *second* Arthurian pastiche I've attempted in the past few months. The first was an SGA retelling of the romance of "John, the Knight of the Lion", (because who other than John Sheppard would go on adventure, get manipulated into marrying a lady, and then go back to his city and get distracted and completely *forget* that he'd married her?) John uses 'you' graciously toward his men and ungraciously toward his superiors but 'thou' promiscuously towards people he considers his equals. Rodney, by contrast, thous everybody indiscriminately until they earn his respect, at which point he switches to 'you' and sticks with it, even while he's calling them idiots. Elizabeth is scrupulously correct, using 'thou' to inferiors and 'you' to superiors and equals, although she tends to fall back to 'thou' in private, even with Caldwell. Ronon uses 'thou' on everyone except those above him in his direct chain of command, because that's how they did it on Sateda. Teyla and the Athosians only use the formal (except in prayer), although she's been picking up the informal a little with people who use it to her, mostly John. Ford uses 'thou' with his inferiors and equals and 'you' with his superiors and has always felt slightly odd about the Major calling him 'you'.
And, hey, since I went there *anyway* - Mulder thous everybody as a direct affront to his aristocratic background, except when he's being manipulative. He was being manipulative when he met Scully, and then never got around to switching back. Scully is very correct in the same middle-class way as Elizabeth, but the longer she spends around Mulder the looser she gets. House uses thou exclusively except with his parents. And with Wilson's wives and Chase's father, and other attempts to be deliberately obnoxious with it. It took him almost six months to get Chase to stop using 'you' on him, and he still hasn't broken him of being polite to Cuddy and the patients. Cameron switched from 'thou' back to 'you' with him after their date, and Foremen always uses you, except when House suceeds in provoking him. Wilson also uses 'thou' with everyone, but somehow with him they assume it's affectionate rather than contemptuous.
Harry Potter uses 'thou' with everyone he calls by their first name and 'you' with evereybody else, except for Dumbledore and Snape whenever he thinks he can get away with it, and Mr. & Mrs. Weasley because they insist, and Slytherins when he wants to make trouble. Bones uses 'you' with everyone, even small children, except the people she considers hers, which is basically just the stooges and Booth. Hodgins considers 'you' to be a tool of the patriarchy; Zack calls everybody 'you' because whenever he tries to 'thou' somebody he gets shot down, except Hodgins and Bones and (when she's feeling generous) Angela. Booth takes 'thou' from Bones and his army buddies but nobody else.
SG1 always sticks with 'you' except in moments of extreme emotion, except for Daniel, who will at the slightest provocation give you a ten-minute dissertation on why English preserved the informal form, and who likes to randomly switch to 'thou' just to see Jack twitch, and has gotten everybody but Jack to use thou with him off-duty. Jack has never yet called a Goa'uld anything other than 'thou', except that sometimes with General Carter he forgets; and he's been on thou terms with Thor since about ten minutes after they met. The Asgard and the Ancients thou *everybody*, but the Nox don't, even when they're calling us children; the Tollan use 'you' but they make it clear it's only to be polite.
Darnit, now I want to write stories where they really do use middle English pronouns. Oh, wait - there's one open on my desktop right now! What a coincidence! I'll get right to that, then.