Here's an example. I wrote the following, and my editor asked me to change it because she couldn't imagine anyone actually speaking this way:
Every so often I get a craving for the kind I can’t find at night. You know the sort of man I mean: a vegetarian Buddhist in thrift-store corduroys, doesn’t drink, rarely pays a visit to the barber. Last time I found one I was coming home on the PATH train at half past six on a Sunday morning; he boarded with a friend, both with twelve-speed bicycles in tow. I knew I had to have him when I heard him say, “You know when you’re riding down a country road and come upon the skeleton of a barn? I love that.” He didn’t notice me then, but I made sure he left his pocket journal on the train...Yes, I heard a guy say that, word for word. I loved how much that offhand remark told me about his personality, and the well cared for bicycle at his side reinforced my impression. I didn't go home with that boy, but Eve sure did.
What can really make good dialogue, though, is all the things that go unsaid. Here's another exchange I scribbled down on Monday afternoon:
HE: So...are you going to be around long enough for us to go out?This reminds me of nights in the study room back in grad school, when Seanan would write two lines for every one line of dialogue he composed: one for what was coming out of the character's mouth, and the second for what he was actually saying. Dialogue—reading it, writing it, overhearing and 'appropriating' it—is one of the most delightful aspects of being a storyteller, and doing it well makes it just as satisfying for the reader.
Something about being on a Greyhound bus gets me really horny. You'll do.
SHE: I don't think so. I'm too old for that shit.
I'll talk to you across the aisle, but there's no way in hell I'm going back behind the dumpster with you at the next pit stop.
HE: Yeah, I don't do the bus thing either.
[a moment later] But I can cook up seafood better than you can buy it at the harbor.
I'm gonna pretend like you didn't just shut me down. In fact, I'm gonna give you one more chance, because I haven't met a woman yet who could resist my crabcakes...