Basically, in Dewford Town, there is a trendy phrase. You can make what's "hip, happening, and trendy" almost ANYTHING. Even "ADULT ANIME".
A female Cool Trainer on Route 111, before she challenges you, says that your Pokemon look like serious actors and that she wants to ask you "for an engagement", which makes it sound like she wants to marry you. It's made even funnier if you're playing as a girl, but in Emerald after you beat her, she only mentions wanting to be friends with more strong trainers like you. Still funny though.
In the museum in Slateport, when you encounter Team Aqua (Sapphire/Emerald) or Team Magma (Ruby), the grunt you fought in Petalburg Woods and Rusturf Tunnel will give you a TM. What's in this TM? Thief, which he claims fitsyourather than him before running off in the middle of his job.
Just before that, there's a line of them waiting patiently outside the museum.
A couple of them even gripe about the entrance fee!
"The air is tasty here."
Many random trainers have hilarious Large Ham tendencies.
GORGE your eyes on this, it's a SILK SCARF!
You know the standard YES/NO means of answering questions in Pokemon? In this game, it extends to asking where you're from. Saying yes will result in the man saying that he's never heard of Yes Town. Selecting no causes the man to say that you've had to have come from somewhere.
"Fishing is all about concentration, you have to focus on the floater"