There are a number of signs that you have finally hired the right entertainer for your event. First and foremost he/or she is more interested in how they can best interact with your guests than how they will be perceived. They should be asking what you want to accomplish, or expect from the performance. If you are hoping for just some diversion for the audience, they shouldn’t be suggesting a stand-up performance that demands all of your groups attention. The ideas and efforts should be directed on your event and how they can best assist in making it the best that it can be.
The arrival time (and they should be early if anything) should match the guests, having the performer ready to begin knocking them dead right from the start. However, if this will be a casually attended affair, then planning for when there are a number of guests there should be the performers goal and suggestion. Remember, they are on your schedule.