I think body language is fascinating. Honestly, you can learn a lot more about how someone feels from their body language than anything that actually comes out of their mouth.
I love applying this to writing because it helps create believable characters. You can tell your reader everything they need to know about a character purely off their body language. Adversely, if your character's body language is wrong, it will hurt your writing.
One thing I try to keep in mind is that often people say one thing, but their body language contradicts them. If you ask a favor of someone, they might respond and say they're happy to help, however you can sense that they feel uncomfortable or unhappy about it—
it's because their body language is portraying how they truly feel.
Likewise, you might have moments in your writing where a character feels uncomfortable, scared, or unhappy, but they try to mask their inner feelings and appear calm and confident. It's important to show small traits of how they're truly feeling during each scene.
If you have a character that's feeling angry, they might stiffen up or change their tone. If someone touches their nose, they don't believe what's being said. If they're nervous, they'll be fidgety, give short answers, and have terrible eye contact. When someone's feeling relaxed and confident, they'll make themselves appear bigger, taller, and raise their hand higher in class. When someone feels vulnerable or scared, they'll shrink by crossing their legs, folding their arms, or sometimes touching their neck.
I've been watching other's body language a lot more closely, trying to take notice of how they react in different situations. My goal is to make each of my characters react believably, thus creating stronger writing.
How do you use body language in your own writing? What popular characters in literature use strong body language?