You do not have to write a paragraph, but this is another opportunity to make a good impression. Ask to see the job description. Most employers should have a copy of the job description handy when hiring. If you just walked in off the street and asked for an application, be sure you know what you are getting into. If there is any part of the description you do not understand or have a problem with, ask to speak with the manager for clarification.
Most retail jobs involve more than just standing at a cash register.
From the moment you walk in the door, have a positive attitude. When you turn in your application, ask the person behind the counter how he feels about his job. Ask if the manager is available and introduce yourself. Even if the store is not hiring at the moment, you can still make an impression by being assertive. Tell the manager what a great store he has and how you shop there all the time.