I'm guessing you mean in the prison service. If so, as a casual, you will generally be escorting workmen around the prison whilst they work. It is up to you to maintain the security of the area, the staff, and most inportantly, their tools. DO NOT let them leave any lying about unattended.
As far as the interview is concerned, be yourself. Think about the questions you are asked, and don't rush into answers. If you are not sure what is being asked, repeat the question back, to show that you understood it, and to give them the chance to clarify it, if needed. Have you any previous security experience, have you done first aid, etc? as these will come in usefull.
A mature outlook on life always goes down well, coupled with a sense of humour, as you need it to work in a prison. Casual jobs can often be used as a springboard to permanent posts, and then there are avenues to become a prison officer by taking an internal test and assessment.
Good luck, and let me know how you get on.