role play interview - HELP!

Well you havent given any information other than its a role play.
Presumably its some form of customer service?
Look at it from the employers point of view as well as the customers.
Understand what the other side wants, listen and then present the options as well as solition.
Be courteous, but firm.
If you dont know the answer, dont pretend , but ask someone who does.
Dont do anything stupid at exposing the company to risk.

When we were doing mock interviews the main pointers we were given were:
Always be confident, Smile, give a firm handshake, and try to maintain eye contact rather than fidgeting.

If it's a job interview style:
Wait to be offered a seat, and always read over your CV before hand because that's probably going to be the main talking point, so make sure you can elaborate on everything on there.
Also make links between experiences and skills e.g. 'I lead my team on the D of E hike' " I believe this shows good team management and leadership skills which could be valuable in this job"

Also try to relax a little, if you're nervous, you make the interviewer nervous too (that's why you smile and make eye contact)
And First impressions are everything so making sure you are well dressed (if not in school uniform) and don't look as if you've just been dragged through a hedge backwards.

It's probably a little late to start for your upcoming interview but I always try to make a note of any time I do something which could be used for my CV or in an interview situation on a word file on my computer e.g. playing at the Christmas concert. Then you have a whole heap of examples for what you can say in an interview and all you need to do is try and think of what these could relate to e.g. Leadership, communication, teamwork.
They mainly ask about Leadership, communication and teamwork, and anything that might be specific to that job, so if you know the situation you will be in for the interview, look over the list and pick out a few key examples for each to memorise so you can bring them up in the interview.