Just want to put forward my experience as a non-pro, from the wedding I shot last year.
It was for the brother of a very close friend of mine. He saw my (landscape and macro!) photos on Facebook, and decided I was the man for the job. I didn't want to do it, and explained every issue that might prevent me from doing a reasonable job. Right down to kit failures and the fact I'm not much of a people person. They still wanted me. I was both a guest and photographer. They covered my hotel and petrol costs, and I got drinks bought for me all night, once the majority of the photography was over.
So, here are a few things I learned from that one event. I don't know if they'll be helpful to you or not, as I guess each event and venue have their own foibles, and everyone has their own style.
Check out the venue beforehand. Make sure you know where the sun will be positioned when the ceremony ends, so you know where to put people for the group shots. Nobody wants to see a load of squinting faces.
If you can speak to the priest (or whoever is presiding) beforehand, then do it. I was so nervous, and he really put my mind at rest after I explained I was a total n00b.
The Best Man (or woman, in my case) should be able to help with the group shots. Gathering people together, and bossing people around, etc. I couldn't have done it without her.
Ask the wedding couple what sort of shots they want. Pretty obvious really! My couple wanted some shots with them pulling stupid faces. Not my sort of thing, but hey, this was their day.
Don't be backward in coming forward. I am an incredibly shy person, but forced myself to be assertive for the group shots. I found that the guests treated me with quite a bit of respect, which really brought me out of my shell.
Not sure if this is the case with all weddings, but the mother of the groom was a right sodding nuisance. Try to bite your tongue if you feel like you're going to scream at her.
With regard to camera settings, I stuck with aperture priority for the most part, and kept an eye on the shutter speed in the church, adjusting the ISO if necessary. To avoid blown dress highlights I underexposed by 2/3 of a stop, knowing I could fix most, if not all, underexposed bits in LR. I shot in RAW, by the way.
If I think of anything else then I'll add it. I don't want to start any arguments. Just putting forward my experience. Incidentally, I never want to shoot a wedding again, but don't regret the learning experience.
