I've worked in many bars, I've been a supervisor and been resonsible for staff training.
If your just going for a normal pub or small bar job, just go in and ask take a copy of your CV be confident but not too much. Remember you will be working with people constantly, so you want to come across as a happy, customer focused, easy to talk to kind of person.
Now if you want to go to, a bigger establishment, more up market bar, cocktail bar, then please do your research!
We like to start people on Friday and saturday nights, you know to throw them in the deep end, so it helps you. its not for our benefit.
we wont expect you to start flipping cocktail shakers, but dont ask us 'which one is lager' 'how do i do a shandy' 'whats cider and black'
It will irritate the hell out of the rest of the staff, you need to be confident and just do it!