I initially applied to Next online for a Christmas Temp position. I received a call 2 days later, where I had an informal interview with the store manager. She asked me a few simple questions and then asked me into store for a team assessment the next day. For this, I dressed in smart black trousers and a plain black jumper. The team assessment consisted of spending time on the shop floor; working the fitting room, the shoe department and the stockroom. This was slightly nerve wracking at first, however if you just throw yourself into it and smile a lot (definitely approach customers and politely greet them, ask if they're okay, even make small talk about the weather!) you'll be absolutely fine, they just want to see you'll be confident and happy to talk to customers. The staff would make notes on your approach to the tasks given and your attitude towards customers! If there's nothing to do, just tidy up and size order the clothes and even make a bit of chat with the staff as it shows you're willing to be a team player and will easily be able to fit in and effectively communicate with the team! After my time on the shop floor (roughly 20 minutes) I was then taken to the staff room and offered a permanent position instead of the Christmas temp position, which was great news and they were lovely enough to give me £10 cash for travel expenses and a £10 gift card for Next! That was a lovely surprise!
My advice to anyone applying would be to just be yourself, and smile a lot, really be willing to show you're capable of following instructions and completing tasks correctly and definitely above all, approach the customers! That's the main aspect I found they looked for! Even if customers don't want to speak, don't be disheartened, just approach another :)
Good luck to anyone with an interview at Next in the future!