This wedding ceremony is about and for the couple. Even though you will be doing the lion's share of the writing, you should never loose sight of this fact.
Consulting with the couple early on is an essential part of the ceremony writing process. After talking to them you will have a better idea as to the tone, theme, length, and rituals involved in the wedding ceremony.
Set up a meeting with the couple, or start an email chain. Things to ask them include:
Having this dialog will also likely inspire new ideas that none of you would have thought of individually. Make sure to stay focused on what is relevant to the ceremony and take notes. Afterwards, confirm which ideas the couple wants to include in the actual ceremony. Plenty of ideas will have been tossed about and it will save you time if you know for sure which ones to develop and which ones to discard.