There are several different types of movie shops.
A checker typically counts the number of people in the theater period sometimes you stay for the movie sometimes you don't . The idea is you're counting the number of people in the theater and it's compared by the client to the number of tickets sold.
There used to be some mystery shop type movie theater shops where you stayed for the movie and evaluated the experience.
The theater installs are the cardboard standies that you see in the lobbies. you don't get to watch the movie for those you just get to come and put together and stand up the cardboard cutouts.
The other type of check involves watching and taking notes of the trailers that played before the movie and most of the time, you are not staying for the full movie.
Hope this helps a little?