This is normally not an undercover assignment. You will be assigned to watch a specific movie and check all of the trailers and advertisements before the movie begins and during the interval duration if any. You will be asked to give a feedback on the quality, message received etc. Most of the managers allows the checkers to remain in the theater and see the film even after the trailer check has been completed.
Audience Reaction checks:
Most of the studios are interested to know (rather they have to) how their marketing department is successful in making the movie trailers or advertisements. In that case, you need to keep track of each movie trailer and advertisement and the reaction from the audience. Some types of assignments requires you to categorise the audience reaction based on gender, age etc.
Seating capacity checks:
The studio heads or distributors may want to have independent count of the patrons who are visiting the theatre on any given day. They may be paid per movie screen or paid per hour, depending on the company. This can help them in understanding the flow and turn out ratio of the audience which could in turn help them in arranging the required number of seats, stalls etc.