Me, I'd plan as much as I could, and that would help guide the discussion.
If you can get access to the venue in advance, then definitely do that, and test lighting / flash aspects and/or natural light aspects if indoor.
Also determine the distance you'll need to be from the subject - to remain somewhat hidden but still get the angles and shots you need.
I wouldn't muck around with primes for the none posed section of the day. You need to be able to react quickly, as certain aspects are only happening once and you need to get them.
Don't expect to be intimately involved in the wedding either, that's the unfortunate side effect of taking on one of the duties. It quickly moves into high pressure work at times.
Later on, when you've got a period alone you can switch to the wide aperture primes and do the fancy stuff, but get the core shots under your belt first.
Getting a solid shot at f8, so everythings in focus, is better than a missed focus f2.8 that has creamy background bokeh etc.
Have someone else, one from each family, heard the groups of people for group shots.
Don't forget the likes of memory cards, batteries, clean lenses, clean sensors etc. Lots of prep to be done in advnace, and have a system where you switch some of the cards during the day, to ensure you don't have all your eggs in one basket.
And have fun