Can you buy tickets at the door?

Yes

If it’s a popular movie 9 times out of 10 they’ll be sold out.

not always. a lot of shows sell out or front row is all that's left

Yes, but it's wiser to buy in advance.

Yes, but it's best to buy them ahead of time online.

Yes you can, but you risk that the movie is sold out, it's best to buy online at their site or fandango