Do you have steps, a live band or DJ with whom they'll need to coordinate with, a special format or program they need to be aware of, etc. Also tell them about the audience. Different demographics will affect the success of the performance. Each entertainer or artist should have a list of questions they ask you. These questions help them to determine what they need to bring and how they should interact with your guests at your party.
Many entertainers have a variety of different skills.
Clowns for example are often times magicians, face painters, balloon twisters and have multiple talents. A lot of stiltwalkers also juggle. Henna has become a very popular party activity. Do they bring a hair dryer to assist in speeding up the drying time. Henna will wash out of clothes with water but don't wait before cleaning the fabric.