How to add character without offending.
To flavor a novel and provide authenticity, authors often use dialect in their written dialogue. But the use of dialect in writing is tricky, and if you don't use care and sensitivity, it may backfire. It may say more about the author and his or her assumptions than about the characters, or it may distract readers to the point that what is being said is overshadowed by how it's being said.
First, let's talk about the difference between accent and dialect. When we talk about a person's accent, we're referring to how they pronounce words. So when Eliza Doolittle sings, "Just you wait, 'enry 'iggins! Just you wait!" she is using standard English with a Cockney accent. Accents are caused by interference from one's native language, as with Arnold Schwarzenegger, who speaks with an Austrian accent, or by interference from one's native dialect, as with Holly Hunter, who speaks with a Georgian accent. Both speak accented standard American English.
Accents In Writing
When writing for a character with an accent, it is tempting to render the character's speech phonetically using nonstandard spellings. However, this practice is risky and should be avoided, unless you specifically want to emphasize how a character speaks. First, there's the question of how accurate to be. The more accurate the phonetic spelling, the more frustrating it will be to read. Most adults read word by word, not sounding words out letter by letter, so forcing adults to sound out nonstandard phonetic spellings would slow readers down, potentially irritating them, and thus distract them from the actual story. Second, if you decide to render one character's speech phonetically, what about that of the others? In Gone With The Wind, Margaret Mitchell uses nonstandard spellings for the speech of blacks while using standard spelling for whites even though the speech of both groups is phonetically very similar. Earl Conrad argues that Mitchell is typical of white Southern writers of her day who employed dialect to reinforce the erroneous belief that blacks are inferior--that their speech is so bad it can’t even be spelled properly.(1) Finally, don't use nonstandard spellings for common pronunciations, such as gonna, g-o-n-n-a, and gotcha, g-o-t-c-h-a. These pronunciations are present in all dialect of English, so there's no need to distract the reader by drawing attention to them.
The other option for communicating a character's accent to readers, which I recommend, is to use standard spelling along with a description of the character's speech in the text introducing the character. One might write, "Her roots in the South were evident in her slow, melodious speech," while using standard spelling when writing out her speech. This method is much easier for the reader and avoids inadvertently stigmatizing a character.
Dialects In Writing
Dialect is about what words are used, how they're pronounced, and how sentences are put together. For example, use of the word pop to mean a carbonated drink is characteristic of the Midwest dialect, the fact that cot and caught are pronounced the same is characteristic of Utah speech, and use of the construction might could in sentences like We might could meet for lunch next week is a feature in the South. Standard American English is itself just another dialect.