Id agree with alot of what is said here so wont repeat. You need to be able to work quickly, but creatively. Like someone said there are no second takes.
The most important thing to me however is this-
Its vital to me that the couple enjoy their day. Remember they are getting married and its been planned for a long time and they are all loved up. Let them have fun, try and not intervene and fry them with getting in their face all day. I've seen photographers take it too far. Yes the shots are mind-blowing but they get back to their reception exhausted.
Remember this and your couples will love you.
I pride myself on letting their day flow how they want it to.
Good luck buddy. When I nail a wedding video, even just one phenominal shot, I find the satisfaction so very worthwhile!