Dear, lovely, fan fiction writers,
I love you, and I love your stories. Many of you have a firm grasp on culturally American English, or have betas that can American-pick your word choice and phrasing to make it “sound” correct when writing American characters.
For those of you that don’t, here is a list of lots of common, easily-fixed words and phrases that throw your characterization off when used for American characters. I present it with love in my heart, this is seriously not a snarky diatribe against British English (which I have attempted in a Doctor Who story before, and probably made equivalent mis-usages), I just want to help.
Straight away = right away
a bit = a little (or in some contexts, “some”, consult an American if you’re unsure)
take away = take out
(it’s) a good job = (it’s) a good thing
half past (the hour) = actual time, i.e., 2:30. Americans might say “half past”, but almost no American teenager would, only older people.
work out (a problem) = figure out (a problem)
(move) about = (move) around
let’s go to mine = let’s go to my house/come over to my house
packet of cookies = bag or box or package of cookies. “Packets” are stacks of papers to us.
fit = hot/handsome/desirable. Unless you mean athletic and/or muscular.
ages ago = “A long time ago” is close, but you should just be more specific or hyperbolic. “That was like, a million years ago.”
Meant to = supposed to. Unless you’re talking about grand fated events like “We’re MEANT to be together!” otherwise it’s “I was supposed to go home.”
Pulls a face = makes a face
Full stop = period
School-specific, especially for public (free) schools where the majority of American students go:
exams (week) = finals (week)
exam = test (a teacher might use the word “exam” on a syllabus, but in casual language, 99% of speakers would say “test”.)
marks = grades
to mark a paper = to grade a paper
teacher’s aide = TA, for both high school and college
University = College, unless speaking about a specific University, i.e. I’m going to the University of Michigan.
I know it’s not always easy to find a beta, let alone specifically an American one, so I hope this helps.
ETA: It also varies for what part of the country you’re in. I’ve lived in Florida, Washington, and California, so this is perhaps most accurate for those places, but I’ve also seen this reflected in American television.
ETA2: Also, in discussing this here and in RL, I think a lot of these things apply doubly to dialog. I’m sure I’ve written “a bit” before, rarely, but I would never, ever use it in casual conversation. And this includes internal dialog, for writing a character, I think.