It really depends. I'll tell you what you do at a good wedding:
You drink a lot of alcohol. You eat a lot of food. You drink more alcohol. You dance. You fall down a few times. You listen to people give toasts. You drink some more alcohol. You go home with a bridesmaid.
If you meant the actual wedding and reception, it again depends on the wedding. Most of the time you just sit there. Stand when the bride comes in. Greet the newly married couple after the wedding at the receiving line. Then kill time till the reception.
Wedding ceremony is typically some kind of affair where the audience sits in seats on either side of the center aisle, then as the ceremony proceeds the groom comes up to the stage, the bridal party comes in, lines up on either side of the stage, then the bride is presented by her father (usually) to the stage and a whole formal affair with vows/ring exchanges/poems/whatever else depending on the type of ceremony (or religious tradition it's based on). Obviously all the folks in question here come down the center aisle between the two sides of the audience, in a simple but obvious presentation. The wedding typically involves a professional photographer who gets paid a lot of money to take all sorts of awesome pictures that will go into a photo album for safekeeping decades later.
Afterwards there's a formal exit with the bride & groom and the bridal party and the bride/groom's parents going out, followed by the audience.
Then the reception is a big 'ole party with catered food & booze (sometimes if the folks are cheap it'll be a cash bar, i.e. you pay for your alcohol, sometimes they pay a lot of money to make it free for everyone). There's a small formal affair where the bride & groom come into the reception area (I guess beforehand they take more pics of the bride & groom together, while everyone is filtering into the reception room) and the bridal party makes some statements about the couple (typically the bridesmaid & best man).
Then there's a couple "traditional" dances, like the bride & groom, the bride w/ her father, the groom w/ his mother, the parents, and a whole lot of people join in for the rest of the dancing. Lots of blackmail material to be had for one with a good camera during this part Especially when there's sufficient alcohol involved.
There's also a ceremonial "cutting of the wedding cake" by the bride & groom.
(this is roughly based off the wedding I attended last night)
Then Spirilis and I steal the plastic cutlery, sweep half a dozen stacks of napkins into my purse ("Love Eternal 2008 - Nicki and Rob!") and then we scam bags of rice, enough for a compliment to our Chinese food leftovers.
That's right. We're classy.
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