I was a department manager for Sainburys on both day and night shift for 3 years, checkout, GM, dry goods and night shift.. Sainsburys are one of the worst companies to work for. They do not give a flying monkey!
In your interview, put forward that the customer is of paramount importance and you'll always follow instructions, but won't be afraid to mention if you think something can be done better. Say you don't mind staying longer as that's what you're expected to do most days (but really do kick up a fuss about it when they take the piss). You should have good communication skills and try and be as chatty as possible. They love a chatty Cathy!
Just be yourself. They are bum holes though. You get paid every 4 weeks. Good luck!