I'd say generally they're pretty relaxed usually taking place in a quiet corner of the pub, they basically go through your application form/cv with you going into a bit more detail and then ask you a few other 'situation questions'.
I think they might of asked me what i know about the company and the pub, if you mention about them never playing any music for a 'better atmosphere' (assuming it's a wetherspoons and not a lloyds bar), promoting real ales and local ciders it usually goes down quite well. There's a January sale on at the minute too so maybe mention that.
They might ask if you've had any bar experience but it's not necessary and they wont ask you any specific questions about drinks or anything, they train most people from scratch.
I don't know if they still do it but they used to have a thing where they tick the number of times you smile during the interview and rate you on how positive you are, strange I know.
Good luck with it anyway, will probably be one of the easiest interviews of your life.