The use of adverbs in english

I just realized that I need to get help with understanding how to use adverbs in English. Let's say that a guy is nice, and he performs every action in that way.

Does he:
a) act nice
b) act nicely

Which one is the correct one and why is it so? Don't be afraid to use grammatical terms.

To me "act nice" means that he's consciously being/pretending to be nice, while "act nicely" means what you originally thought it does: performing every action in a nice way. "Act nicely" isn't something native speakers would say, though. They would usually phrase the idea differently.