I had a christmas job at John Lewis, from November till end of January. I was working in the coffee shop as a catering assisstant (which is a posh term for SLAVE), so I just had a one-on-one interview. It was a b*tch of an interview though! I was asked some typical questions, like why did I want to work for John Lewis and what did I think would be expected of me in my role, etc. But there was also loads of random questions, like "if I was to ask your friends what you were like, what would they say?" I somehow ended up telling her about the time we all went to Fuengirola and my mate tried to throw a bucket of water over me from the balcony. I'm sure that story went down well. I was also asked how my future career plans (journalism) related to the position I was applying for. Er, in no way whatsoever!? But, miraculously, I managed to bullsh*t my way through the interview (and beat off some stiff competition, apparently) and got the job. I then went to an induction day with people who had been hired to work in other departments of the store as sales reps. I got talking to them, and some of them had been to a recruitment day, and been picked from there, which sounds like what you'll be going to. If you think the recruitment day's bizarre, just wait till you get to the induction day! You have to watch a load of videos, and go on "mystery shopping" adventures, and make group presentations, etc. There's unlimited free water and Opal Fruits though, I made sure I stocked up on the Opal Fruits!
OK, I've waffled on a bit there. I don't know whether you've already been or not, but good luck. John Lewis is a really good company to work for, they treat the staff, sorry, PARTNERS, really well; there's great staff benefits and facilities. Just don't work in the coffee shop, it's HELL ON EARTH. Seriously.