One common problem I notice frequently when I’m critiquing and judging contests is that the character’s speaking voice doesn’t match his “thinking” voice.
His thought patterns may make him sound very American and educated -- probably because that’s what you are as the author. Then he opens his mouth to speak and I’m shocked to discover that he’s a street thug from England. Even worse is when the author who struggles with grammar usage tries to write a character who must be well-educated to achieve the level of professional success I'm told he has. So when that neurosurgeon from the Mayo Clinic says something like, "I seen them yesterday," you've lost me.
Remember that unless you're writing in omniscient point of view, the narrative in your book is nothing more than silent dialogue in the character's point of view. His thinking voice must match his speaking voice. If he's a street thug when he speaks aloud, he's not likely to think in long, complex sentences when he's noticing his surroundings.
He's not going to think about the obviously inexpensive sedan in the driveway, he's going to think of it as Daniel's cheap-a## ride. Inside the house he's not going to notice body odor, he's going to think it smells like s##t.
Likewise, his dialogue must ring true for who he is. A member of the military on deployment in the middle-east isn't likely to say, "Gee whiz, Castor, the computer's gone!"
I'm not saying that you have to use language that you personally find offensive, but it's often better to skip the expletive completely than to substitute something a person in that situation wouldn't actually say.
As I've said before, keep in mind the roles the two characters play in each others’ lives. (This is one reason secondary characters are so vitally important to the success of your book.) The Lord of the Manor is unlikely to have a heart-felt conversation about his dead child with a servant, but I can't even begin to count the number of scenes I've read where he does just that.
To avoid tons of internal angst that might put the reader off, he needs to have the conversation with someone, but make it someone he would actually share the information with -- or motivate his decision to share it with an unlikely person so well that even your harshest critic won't be able to find fault with his decision.
The hard-nosed detective probably won’t share his true feelings over his divorce with many people -- but he just might occasionally break down to the bartender at his local pub.
Many characters will seem naturally reticent when you begin to work with them. Three-quarters of my characters (especially the heroes) begin life wanting to protect their privacy, even from me. But again, if you don’t find someone for them to have conversations with, you may lose readers during your characters' long bouts of angst-ridden introspection.
In short, when you're writing dialogue, the most effective thing you can do is to think through the set-up logically using your internal editor -- that skeptical side of yourself. Once she's satisfied, then turn your creative self loose. Just remember to remove yourself and your goals from the scene as you start writing. Scenes often go wrong when we fail to get ourselves out of the way.
Once the dialogue is written, send your creative side away and read the dialogue aloud. Read it to a spouse or a child. Read it to the dog or the cat. Read it to the wall if that's all you've got. But read it out loud and let your internal editor go to work again. If you've written a sentence without contractions, but those contractions feel natural when you read the work aloud, change the work. If you skip words, cross them out. If you add words, put them in.
Using a different sense to edit your work than you used to create it can be an effective way of smoothing out the rough spots.