It is important to plan ahead for musician set-up prior to the wedding service. Below are key questions to ask the musicians you hire.
Will you need to rent a piano or does the keyboardist have one he can bring? How about a small PA system for guitars, keyboards, vocals, etc.?
What space and power requirements do the musicians have?Will they require any sort of lighting? (playing in the dark is sometimes difficult and people will not be able to see the performers)
Do they know exactly what to play when?
Do you have a backup plan if there is an emergency, a musician is sick or a vocalist can not sing?
(the performer can usually suggest an alternate, and hopefully can deal with such a situation professionally)
If the service is to be outside, is there a tent they can setup under in the case of drizzle, and is there any type of heat lamp if the temperature is below 60?
(even a light drizzle can be very bad for guitars and electronic gear, plus any temp below 65 tends to make the fingers cold, and performing harder).