I've been thinking about the "show, don't tell" motto that writers have been told since forever. Breaking it down makes sense, but here's what I've discovered about the advice: It forces you to think more about your characters.
Here's what I mean: Pretend you're writing a scene and just want to get the basics down. Let's say it's a fight between your two main characters. It might look something like this:
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"I can't believe you took the money without asking me!" John felt betrayed and hurt. "You know we were saving it for something important!"
"This was important!" Madison was annoyed at John's reaction. He should realize how important this was to her. "Besides, I get paid in a few weeks."
"That's not the point! You should have discussed it with me first." John couldn't believe how she was missing the point. "How can I trust you now?"
"Trust me? What's that supposed to mean?"
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You get the idea. It's pretty bad.
Now let's employ the SDT magic. You're forced to give a little more background and establish the setting. Where are they fighting? What time is it? What are they doing during the scene?
Since you're using SDT, act like you're a director creating this scene. As an actor, I would say, "Okay, I get this character is upset, so how should I show that?" I'd expect you to say something like "Cross your arms across your chest, maybe step back so you're using body language to show your emotion. Raise your voice. Think about how you act in a real life argument. Do you look right at the person, or do you look away? Does your voice change?"
This sort of information can get the ball rolling on your character. You can picture them a little more clearly. Let's say John's voice changes, gets quieter because he's so hurt. His eyes will widen a bit, and once the argument heats up he'll stare right at Madison, wanting answers. Madison, however, wants to deflect the entire fight, so we'll assume that she uses her hands a lot. By the end of the scene, she's feeling defensive. How can we show that? Body language would be best, so maybe she'll cross her arms against her chest, something people usually do when they're feeling defensive or territorial.
Guess what - you just came up with little habits for them.
So after sitting in your director's chair, you let the scene unfold again, with a little more SDT:
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John stared at his wife. "I can't believe you took the money without asking me!" He stepped forward, his throat tightening. "You know we were saving it for something important!"
Madison frowned at him. "This was important!" She turned and shrugged off her jacket, tossing it on the sofa. "Besides, I get paid in a few weeks."
He shook his head. "That's not the point! You should have discussed it with me first." John slumped on the edge of the sofa, looking at the floor while she removed her shoes. After taking a deep breath he looked up at her, his eyes wet. "How can I trust you now?"
"Trust me?" Madison crossed her arms across her chest, her eyes narrowing. "What's that supposed to mean?"
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A little better. Clearly John is more hurt and Madison is more on the blase side. If you want, you can make John more aggressive, and Madison more wounded, still using SDT:
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"I can't believe you took the money without asking me!" John narrowed his eyes at his wife as she set her bag on their bed. "You know we were saving it for something important!"
"This was important!" Madison forced herself to look away. She cleared her throat and rubbed at her arms. "Besides, I get paid in a few weeks."
John huffed. "That's not the point! You should have discussed it with me first." His lips curled back in disgust. "How can I trust you now?"
"Trust me?" She lifted her gaze to him, blinking back tears. "What's that supposed to mean?"
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There you go. Is it a great scene? Nope. But hey, now you know that in the first scene, John's a sensitive guy when it comes to trust (nearly crying when confronting his wife about financial infidelity). He also tends to feel helpless (slumps on the sofa) in such sitations. Madison, however, shrugs it off (casually removing clothes, changing the subject) but doesn't like accusations of being untrustworthy (arms crossed, glaring).
In the second scene, you learn that John's angry and repulsed (glaring, sneering) by Madison's spending. She, however, feels threatened (looks away, rubs her arms) and hurt (trying not to cry).
Sometimes it helps to think like a director. In my current work in progress, my character rubs at his forehead when he's stressed, whereas another character bites at his thumb. I have to be careful that they're not doing it too much, so if I find they're doing it too much, time to play director and find other ways to show that they're stressed. I might rely on past experiences or watch TV or movies to get ideas. The important thing is to keep the actors/characters busy. Dialogue only takes you so far.