You know when you see them talking to each other, and you don't get a chance to listen in? That's because they're trading something with each other. Either a catchphrase, or a greeting, even a shirt- those are fun to see happen. Yeah, they've been doing this since the original game on the GameCube. That's why starting in Wild World where you could trap them- you couldn't really trap them on the original because after you went a few screens over, the holes would vanish. If you trap them, they'll never talk, and never swap anything, and you'll also know where they are if you're wanting pings from them, but if you like the randomness like I do, let them roam free, but be warned, you may have your entire villager all calling you idiot (I figured that catchphrase would catch on)!
Annika: I gave a cat villager the phrase 'meow' and than a wolf villager stole it... a cranky wolf villager... what are you doing there, Fang. You're not a cat...