Can you just walk in and pay to watch a movie or do you need to pay for tickets before hand

You can but i don't think that you'll be able to choose your seat. I suggest ordering your ticket thru AMC app or Fandango beforehand to check any available seat. With this pandemic, there's nothing more awkward and uncomfortable than sitting next to a stranger in a movie theater when there are still a lot of empty seats around. lol

Either, but if you walk in, you're stuck with the seats that aren't reserved. You can reserve and pay for seats in the AMC app an hour or two before the movie, or from the parking lot (if you want to see what's still available)