Dialogue is the thing I have the most trouble with when writing. My natural instinct is to make it grammatically correct with the correct syntax, or to ramble on and on, making it too 'pretty' to be realistic. Over the years I've discovered that this simply doesn't work. Not only is it too jarring to move from smooth descriptions to unnatural dialogue for most readers, it is uncomfortable too. It detracts from the story, from what the characters are saying, and then readers become lost and once they've wandered away from your book's path, they don't tend to come back.
So, dialogue that is too formal is out.
But at the other end of the scale is dialogue that is far too informal. It may be exactly how people actually speak, with contractions and pauses and missing words, but reading it is a maddening experience. It works when said out loud, but not when read in one's head. If script-writing is your thing then it's perfect. Otherwise, it's annoying.
By coincidence, I recently read two books in a row that incorporated these traits. The first, with the most informal dialogue that I've come across, was The Bodies Left Behind by Jeffery Deaver. I dug in to my first Deaver novel with high expectations, but something was making it slow going. Eventually I realised that although I enjoyed the descriptive passages, every time it came to conversations between characters I had to really concentrate on what was being said. It was no longer a simple matter of reading; now I had to translate from 'natural' speak to 'written' speak. And sometimes that took a while. It wasn't the sort of book I could read when tired, that was for sure! Passages started to be scanned rather than read and although I did finish the book (eventually), I just wasn't satisfied. It hadn't been a fun reading experience.
The next book I took from my 'to be read' shelf was Crisis by Robin Cook. What a difference! Now, instead of informal speaking I was thrown into what seemed to be a Victorian novel, very proper and correct, even though it is actually set in 2005. Again, the descriptive passages were entertaining enough, but rather than being drawn into the story through the dialogue, I found myself being amused by the way it was written. I'm sure that wasn't Cook's intention when he wrote it. So, once again, although for different reasons, dialogue was skipped. I must have missed important revelations for the plot, but it was just not a comfortable book to read, so I didn't really care.
At least I'm not the only one who has trouble with dialogue!
But how to combat the problems? How does one find a middle ground between formal and informal dialogue? It's difficult, but if it keeps the reader interested then it's worth it.
The best way I have found is to just write it. That's the first step. Write your short story or chapter or 1000 words or whatever. Once it's written, and the dialogue is in place and says what you need it to, read it aloud. If it sounds like you've written a speech, or you're speaking to Mr Darcy, take the hint. It's not right (unless, of course, you HAVE written a speech, or your writing is set in the past...) and needs to become less formal. Try swapping some words for slang, or contracting, or deliberately messing up the syntax and see what happens. Change it as many times as you need to until it's right - it may not seem important, but it most definitely is.
If when you read it aloud it takes longer than it should, it's probably too far the other way - too informal. That's because, although when we speak we automatically go for the easiest route, when we read aloud it's a different matter - what we see and what we expect are different things, and this can cause a slow down. The brain is trying to process what it's seeing and what it's saying, and it's having problems. This is another sure sign that something is wrong with the dialogue, and that something needs to be changed. In this instance, try taking out some of the slang and contractions. Let your brain read what it wants to read, rather than what's there, and you should end up with a more comfortable reading experience for everyone.
My final bit of advice regarding dialogue: it's just as important as the description. It can reveal so much about your characters and the plot and it shouldn't be underestimated.