A six-year-old visitor got me thinking about syntax and speech patterns. So much of personality is conveyed in how we put our sentences together, and if a writer can capture unique syntax for her characters, they sparkle and stand out, distinct from each other.
With children we might see, as I did, an adult pattern copied unconsciously by the child who lives almost exclusively with polite adults. As my little visitor went outside to play, he glanced at the table where I'd set out refreshments. "Have yourself a brownie," he said casually, as if he were the host and I hadn't made them and put them there. Not a unique statement by any means, but in the mouth of a six-year-old boy, it reveals a lot about what he's seen and been taught.
In MACBETH (one of my favorites, you know by now) there's a telling comment from the Lady. When informed that Duncan has been murdered she replies, "What, in our house?" Again, not an unusual comment, but revealing of character. Not, "Oh no!" Not "Say it isn't so." A character whose words reveal a selfish slant, for whom everything comes down to what-does-this-mean-to-me.
Capturing foreign or regional idiosyncracies is great for character portrayal, and teen-speak and slang can reveal either sloppy thinking or crystal-clear imagery, depending on usage. Swearwords tend to indicate dull-wittedness, because they require very little originality. However, a certain old man of my acquaintance could string them together quite creatively when his cattle didn't respond to his commands. Not that the cattle appreciated it, of course.
The key in dialogue, then, is to make each character speak for himself. He can't be you talking, and he can't sound like everyone else. If you listen to those around you, you will hear a cadence to their sentences and pick out unique phrases and syntax that individualize them. Your characters need that same uniqueness, need to say what they have to say in a way that makes them real.