Last week I spoke about the importance of adding physicality to dialogue, because people tend to communicate as much (if not more) with their bodies and their actions as they do verbally. In fact, it might be helpful just to think of dialogue as another variant of action: your characters are in the act of "doing" communication, and that communication should both physical and linguistic forms.
One of the things that's critical, I think, about showing the interplay between what a character is saying and how a character is behaving is that it often enables you to show contrast or tension between the two. People often don't say exactly what they mean. Think of all those times you've been around your secret crush, and not a peep about your abiding love for him/her has come out! Think of all the things you've wanted to say to your parents, or the times you've accidentally let slip a cruel comment to a friend whom you resent for one reason or another, etc etc.
So think of it like this: You have a variety of ways to show action. Your character can "do words", and your character can "do behaviors"--cut up cucumbers, look away nervously, chew fingernails, etc. It's the interplay between these two forms of "doing" that allow you to show the reader your character's complex feelings, characteristics, and emotions.
But how much is too much? Pamela Harris rightly points out that too much physical description in the middle of a spoke exchange can pull the reader out of the moment, and she's absolutely correct. However, I think there are three basic principles that can be helpful when determining how much or how little physicality to include:
1. The key is "interplay"--make sure what your characters are doing is meaningful. This applies, of course, to both the doing of words and of behaviors. In real life, you have exchanges all the time that are meaningless, in the sense that they just pass the time and don't become relevant. In books, every spoken exchange must either advance the story or our sense of the characters (preferably both). Similarly, your character's behaviors must be relevant. Don't just write in little physical details for the sake of it.
In other words, DON'T do this:
"I really like you," Debbie said, while cutting an orange in half.
WTF does an orange have to do with her confession of like?
But DO do this:
"I really like you," Debbie said, scuffling her feet.
Okay, you should do something better than that, but at least you know that the fact that she is scuffling her feet connotes nervousness. In other words, because of the interplay between the words and the behavior, we learn something additional about the character, without having to be "told" that she is nervous.
2. Think about real-life situations in which you would notice someone else's physical behaviors, and real-life situations in which you wouldn't. Adjust accordingly.
For example, if you find yourself at a party, sitting next to your long-time crush and having the first spoken exchange with him you've had since he got cute in sixth grade, you would no doubt be noticing everything about his body language. Is his knee brushing yours? Is he tapping his foot? Biting his lip?
OMG does that mean he likes you????
Okay, you get the point.
On the other hand, when you and your best friend are screaming at each other because you've just discovered she has been secretly hooking up with your longtime crush, you no doubt wouldn't care that there was a lock of hair hanging down over her right eye or that her fingernail polish had begun to chip. In fact, you wouldn't care about anything about anything except pushing homegirl off a cliff.
Your writing should reflect the emotional intensity of the moment--people tend to lose perspective and a sense of detail when they're extremely angry, or terrified, or overwhelmed with joy.
3. This isn't so much a tip as a truism--remember, it's much easier to cut physicality down than to add it later. Part of what focusing on physicality does is force you, as the writer, to vividly imagine the moment--to slow down and truly understand what your characters are thinking, feeling, doing. It forces you to understand them better so you can better evoke them for the reader.