Okay, so today, my villager (Punchy) was walking around with a frown on his face. Has this happened to anyone else? I'll put up a picture once I'm home.
They also sometimes have a 'thought bubble' above their head because they're thinking. Villagers talking to other villagers can cause this and many other common ones, such as anger, sadness or happiness.
Yes, I have seen this happen a few times after they have had a conversation with another villager (and sometimes the other one walks about happy, seems pretty mean!). It doesn't last for too long though, and he should be back to his normal mood soon.
It is when they talk with another villager, even if you miss the conversation they'll have special emotions, they can be jolly or frustrated ^^ Also when they seem to be thinking, it's generally that they sh... shaked... shooke? What's the preterit for 'shake'? Anyway, when they don't see anything falling from a tree x)
I love it when they get angry and then frown and stomp around. I feel bad for laughing at their anger but I just can't help it sometimes.
I think it's from talking to another villager. As the person above said, they can get emotions from that even if you don't hear their conversations. They can also change shirts to what the other villager is wearing or pickup their catchphrase.