I think I've mentioned before what an effective tool eavesdropping can be for getting authentic dialogue. That's not to say that you should include, verbatim, every conversation that you hear, but heading down to the mall and just listening to the way teens (or adults, if you don't write YA) talk to each other can really help you write more authentic dialogue. Take the best of the best of what you hear and stick it in your book.
But lately, I've started a new (and much creepier) kind of eavesdropping - observing for emotional action. As I go through my MS, I'm focusing on getting rid of any of the instances of "telling" and making them all "showing" - for a basic example, changing something like "fear rose up in me" to "a chill ran up my spine." Taking the actual emotional word out of the phrase and focusing instead on how the emotion makes the character feel - the character's physical reaction to the emotion - makes some of the bumpier scenes better.
Usually, I get the ideas for emotional reactions by thinking about how I would react in those situations. I close my eyes and imagine a time when I was scared, or happy, or nervous, then think about what my body was doing. Of course, that doesn't always work because 1. I'm not my characters, and we don't react the same way, and 2. Eventually, I run out of reactions, since I tend to do the same things. So I've started looking at the way people react physically as well as their dialogue.
And I'm getting so much great material!
Yesterday, I was in my local Super Target grabbing a coffee, and two women nearby were discussing the recent divorce proceedings one of them had gone through. They were going through the judge's or lawyer's report line by line, reading it out and reacting to (and dishing on) and it said. (Why they chose the cafe in Super Target to do this? No clue.) It was fascinating to watch, because things had obviously gone in the woman's favor - she was jiggling her leg, clasping her hands in front of her smiling mouth, and looked as if she were going to explode out of her chair at any second. I took my time pouring my cream and sugar into my coffee so I could watch their excited reactions and make a mental note of their physical clues, then when I came home I wrote the actions down in my writer's notebook. Now, when I need a character to act happy or excited, I can refer to my list, and maybe, if it fits the character, she'll be modeled after the woman in Starbucks.
So, yeah. I'm a creepy stalker. But hey. It's for art. And it totally works.