The dress code.
In order for proper event photography coverage to work, the photographer needs to be properly dressed for the occasion. Events can vary a lot. Some are fairly casual while some are so formal that they can be categorized as black tie events. Whatever type of event you may be planning to work out on bringing to fruition, you need to make sure that your event photographer is properly looped in on what it is all about. Tell him about the dress code. Show him the actual invite for the event. This way, he more or less has a bit of an idea about the dress code that is being required of him. Stealth and the ability to move in and out of crowds without attracting too much attention to himself should be the main goal of event photography and this is the type of thing that you ought to start working on when you are coordinating with an event photographer of your choice.
The most common and fool proof outfit for someone who works in the event photography vertical is for him to dress up in all black. Black is one of the most inconspicuous colors out there and something that will serve your photographer well. They probably know about this already but it sure does help if you have an actual discussion about how you would like him to show up and dress up during the time of the event.
The three-click rule.
Your guests will be there and keen on being able to participate with the program. Sure, they would appreciate a few photos or so but they are not there solely to get their photos taken. With that in mind, it would be wise for you to make sure that your professional working on the event photography shoot understands the value and importance of brevity. He needs to be shoot well composed photos in a swift and decisive manner in order for something like this to work out for you in the first place. Have him shoot 3 clicks in a certain scene and then have him move on to the next scene or to the next person or group of people. Making it short and sweet will be the best way to go for as far as event photography is concerned. So get him to check it out accordingly and to follow said rule.
The need for an assistant.
Depending on how large the event will turn out to be, your event photographer needs to be fully prepared for it. If you have a guest list of more than 100 people, for example, one event photographer might not be enough. He will not be able to cover as much ground as he needs to when he is shooting photos of your event. Let him know if he might need an assistant shooter to help him out with the coverage of the event in order for it to pan out smoothly in the end.