For those who have read my stories, I don't always set them in the heart of the good old US of A...in fact I hardly ever do that. Most of my stuff takes place in Fantasy worlds, namely "Tereand"; nevertheless a lot of the characters have notable earth accents in my head. Sadly this is one of the most difficult things to write in a way that ensures people don't slam the book shut in disgust.
There are a few reasons to use phonetic accents:
a) It minimizes the homogenous feel you get in Fantasy that every one sounds the same and speaks the same language. This is especially the case if there is more than one race/kingdom.
b) It gives bits of information. We know immediately whether a person is: educated or not; what social status they have; or how much money they make.
c)It helps identify who's speaking.
Unfortunately it also provides a good chance for you to bugger it up with some truly cringe-worthy dialogue.
a) Loss of clarity: Sometimes if you're too painstaking about a written accent it obscures all language.
ie: "Tha quek broown falks jemped o'er the lei-zee broon dock."
It would require a fantastic ear and a lot of patience to unravel that the quote was actually saying "The quick brown fox jumped over the lazy brown dog." For most people it's not worth the effort.
b) Annoying people: I don't know about you, but when I was a kid I read Justice League Europe a lot. One of the characters that drove me MAD, however was "The Crimson Fox." She was this femme fatale french woman in a red bodytight. And all of her speech: "Eet looked like theez." After three issues or so I was tempted to go through her bubbles with a red correction pen. And that's the thing. We know she's French. She never shuts up about it. So the accent is superfluous to anyone who can picture a heavy French accent in their head.
c) Just plain ambiguity:"De moornin. She be coomin up ovur de ocean."
POP QUIZ: Is this person Dutch, Caribbean, Dutch Afrikans, Scandanavian, or New Orleans? You don't know do you? You'd be surprised how often an author will introduce a character and not include information to justify an accent like that. If you're in a Fantasy world where there is no Scandanavia or Holland, it's even more confusing!
d) Spell check: This is more a warning to authors than readers. Remember that any baroque embellishments you put on the English language will haunt you again in the dreaded "SPELL CHECK" stage.
So...how does one avoid these pitfalls, especially if we want to put our own flair on a character? Damned if I know but I've found one or two sure-fire methods that can make a reader hold his lunch.
1) The dropped letter: USE SPARINGLY. When used with just enough moderation you immediatley get the impression of a hempen home spun. Use it too much and your character is suddenly an amateur theatre production of "My Fair Lady."
Ie: "I went 'round the 'ouse but there weren't nobody there," is okay.
"I 'id it in the 'ole h'around the 'orseman's 'ouse" will make your editor try to shoot you.
2) Cultural idioms and slang: "Come and have a go if you think you're hard enough"
I've never heard any American say this. A phrase like this is distinctly British and therefore, if the reader knows this, they will hear it as British. Little phrases like "F***in' Ay" or "She's a cold one, yah?" or "Eh-yup," will conjure up certain dialects. But you have to be thorough on this one. Really listen hard to a dialect, try to pick up inflections, phrasings, go there on a vacation if you're solvent enough. Make sure that a character who uses the phrase "Pull the other one," doesn't later brag that he's "smart" (clever) or say that someplace is a "dump" (tip). If you have a friend from the country you're trying to emulate, let them have a look at your work. They might catch things you've never dreamt of.
3) Describe it, don't write it: I have a character who talks with a Welsh accent in my head. But I've never written it out because Welsh is very hard to transcribe phonetically. Also my story takes place in a world where no one has heard of Wales. So instead I describe his voice as "a musical tenor" and that he speaks "in lilting tones, as if singing." That's really all you need to get the idea across.
In a lot of cases, I just have to let go. My main character, Linus, is from two worlds (Human and Elf) so in my head he has both accents and sounds like an Ex-Pat like Terry Gilliam or Christian Bale. There is NO WAY I can get this across so I don't even bother. I just have to let people imagine what he sounds like as best they can.