Of all the literary pet peeves I have, poorly written dialogue is probably the biggest. I will forgive typos (as long as there aren't so many the prose is unreadable) more quickly than I will forgive characters who sound like robots--you know, assuming they're *not* robots *G*
In preparation for writing this blog post, I went looking at other people's advice on the subject--that was less than ten minutes ago, because straight away, I found advice that I disagree with, in the strongest possible terms.
"Use juicy verbs, edit superfluous words and keep sentences simple. Reveal complex characters with simplicity. Again, often what’s not said that is most important and revealing. Most “real” speech contains fragments, “ums” and idioms. Don’t include those. Don’t have your character say something unless it’s pertinent to the story or the character."
I agree with the very last sentence: Don't have your character say something unless it's pertinent to the story or the character. The rest of it is rubbish. Real people speak with superfluous words, we us "um" (although too many I written dialogue is going to be tedious for the reader), we sometimes speak in fragment sentences, we hardly ever use "juicy verbs" (i.e. the average person doesn't stop to think "I shouldn't use passive voice when I tell mom what happened to her vase; I should avoid copulas whenever possible, substituting instead action verbs), and we most definitely use idioms. The idioms your characters use will clue readers into their age, ethnicity, and what part of the country they're from. We also um and erm, and let sentences fall off part way through, and sometimes we go round the mulberry bush instead of getting straight to the point.
There is a fine line, of course, between real and boring. One of the best pieces of advice I've read sums up that fine line like this: dialogue doesn't repeat real conversation, it imitates it. It cuts out the boring parts, many of which can be summed up in exposition. (E.g.: they said their goodbyes and Tom disconnected the call.)
When you're writing exposition, all that stuff that comes in between dialogue, definitely use the best verbs possible, avoid idioms (unless they fit he situation), and avoid over-using fragment sentences. Almost always avoid passive voice and replace as many copulas with action verbs whenever you can (again, opting for what works best for the text, not what works best for your English professor.) When you're writing dialogue, the ideal is to strive for speech that sounds natural but won't bore your readers to tears (or worse, bore the acquisitions editor into tossing your ms into the circular file).
In real life, my husband and I might have a lengthy and time consuming conversation about dinner. I can tell you from experience that those conversations don't serve to move our personal plot along one bit. If we were characters in a novel, the first time we had that discussion it might provide some entertainment and certainly some insight into our personalities. (We're both to bloody indecisive for our own good and each of us is usually hell-bent on pleasing the other--unless I'm having a mad craving for Mexican food *G*). So when your characters talk about dinner plans, it needs to serve an actual purpose. Has one character forgotten the other's food allergies? Or is he being a prick because he knows the other guy doesn't like Chinese, but insists on it anyway. Or maybe he's the one who hates Chinese, but he knows his best friend loves it, so he suggests going to Chang's for her birthday.
It's also important to remember that characters are people, that means they sound different from one another, some to a greater degree and others to a lesser. If you take two twenty-year-old males from the same neighborhood, they're probably going to use a lot of the same words. If you take a poor twenty year old from the inner city and place him next to a twenty year old from a wealthy white-collar family living in Beverly Hills, he's likely to make different word choices and have different speech patterns. Education, socio-economic background, culture, ethnicity, and age all play into the way people talk.
The best way I've found to learn about speech patterns is to eavesdrop on conversations. I know. It's rude. But I'm always listening to conversations going on at the other tables in restaurants or to what people are saying to each other as we wait in line at the bank or the grocery store. Mostly, I'm listening for words and syntax more so than actual content--but sometimes the content is pretty inspiring, to! I also credit writing fanfiction with helping me develop an ear; I wanted my version of other peoples' characters to sound true to form. (That's one more reason why I disagree with the people who say fanfiction writers are lazy. In some ways, I think it's harder to write another author's characters accurately than it is to create your own from whole cloth.)
A couple of the "tricks" I employ when I write is to pick a couple of words and always have a character use that word. As an example, in my current (and currently neglected WIP), I have two main characters, Dillon and Andy. Andy is a high school drop out and Dillon is college educated. But Andy is very well read, especially for his age (and those reading choices have been explained in the story), so sometimes he uses words that the average eighteen year old wouldn't choose. He's still a street kid, so when he's talking about his favorite recreational drugs, he uses "molly" and "pot." Dillon, an attorney, refers to Andy's bad habits as "MDMA" and "marijuana." On a deeper and more personal level, Andy always uses the word "stupid." Dillon always uses the word "dumb." Because they're both typical Michiganders, they both drink "pop" not "soda." They both usually use grammar that wouldn't make an English teacher cringe, but of the two, Andy is more likely to use the less correct word (although very few human beings speak with correct grammar 100% of the time.)
Here are a few good articles I found on writing dialogue: