The whole purpose of the interview is to determine whether
a) The candidate is suitable for the position that you are recruiting.
b) That they will cause you as few difficulties as practical.
c) That the details on their application form/CV are accurate.
It is also an opportunity for the candidate to see if they like you, your business, and to ensure they will be happy with the working environment.
I have zero experience in retail but I would assume you need to find out:
? Are they open, friendly, and able to communicate well?
Perhaps you could explore their reaction to offering them a brew. Do they respond politely, or is the response mumbled? Do they have a nice open body posture or do they appear closed with hands clasped and arms crossed?
? Identify and explore relevant work experience
Do they respond coherently and professionally?
? I would assume in retail that a basic level of numeracy is required.
Maybe you could offer a quizz: "A customer comes in and orders a Coffee and a cake. The bill comes to £4.25 and they give you a ten pound note. How much change do you offer?" If it takes too long for the candidate to respond £5.75 then you're gonna end up with a long queue, slow transactions, and unhappy customers.
? If the candidate is working with money, they will need to be aware of how to spot forgeries. Ask them what they know about fake money and what should be done if they are given counterfeit money. Also you need to identify if the candidate knows how to use credit card machines, and what to do if a transaction is declined.
? Dealing with problems. Ask a question that determines how the candidate would deal with an unhappy customer. You want to work out whether you are going to have to come running every time a customer is unhappy, or whether the candidate has the ability to troubleshoot. You could make another question: "A customer complains that his coffee is cold. What do you do?" or "A customer complains that you have given the wrong change. How do you handle this?"
There are surely other things relevant to your trade and retail in general that I may have missed but these should be some handy pointers to get started.
Throughout the interview, you want to analyze whether the candidate is giving off good vibes, after all you want someone who is positive, upbeat, friendly, and doesn't constantly hassle you right?
Take your ipad/tablet/pen and paper and make lots of notes. If you are interviewing many people, I can assure you will forget things. I've handled many interviews where 2 people were perfect, and upon reviewing the notes suddenly remembered a very relevant point.
Finally, do not let on that you are in any way nervous. Be upbeat in your tone and frame of mind. You are offering a job here - you are in charge!
Good luck :)