Let's say you want your audience to know that this girl is falling for a guy. Her character is already well-established as more of a tomboy, dressing in a funky wardrobe.
A Little Boyish
All you need to do to show that her feelings are changing is to get her to choose to break out of her normal style. Dress like someone she isn't. Do something she wouldn't do otherwise.
Lip Gloss While Waiting
This will work for guys as well. He normally bum around in shorts and flip flips? Put him in a tie. All you need to do is break character enough to be uncomfortable but excited.
Then, if you want to break your character's happy anticipation, do something to dash all hope. Then have your character undo what was just done...
Wiping Off the Gloss
It certainly wouldn't hurt to have your star give a few death glares now and again either.
Your Media Production Mentor