Its easy to get confused - and I know you are already nervous - but an informal chat is still an interview- they will always be assesing what you are doing- key point is to relax
I had 2! informal meetings before my actual interview!
The nice thing is that they are much more relaxed, one on one, and you should try to act yourself - keep in mind its an interview
The only thing you really need to know is why YOU want to do it- what you hope to gain - and try be realistic not say what you think they want to hear - you should know why you wanna do this (even if its not 100% true)
you will find that the supervisor should do the most talking- most scientists do
it'll help if you have an understanding of the field- don't go nuts on the specifics but make sure you have know the field in general it makes it easier to listen to - most supervisors don't want you to quote their papers ad nauseum
enjoy it and be glad you are getting the chance to met them first
Good luck
S