One of the most important duties...

I managed to find another Bridesmaid blog (I swear, I searched for them before I started this and failed), and found something that struck me that I definitely should share:

"When you agree to be a groomsman or a bridesmaid, consider that you’re not just agreeing to stand by that person as they get married. You need to stand by them even if they want to get in your car and drive away. No questions asked." (found on Bridesmaiding.com, a post about the role of the Best Man)

This is important. Even if the bride or groom have spent all this money on a wedding, it's best to make sure they're ready to be married to each other. Because if the answer is no, and they need that getaway car, it's sparing them the ordeal of a divorce later. Once they're married, it gets more difficult to get out than just before the wedding.

So maybe the best time to mention this to the bride or groom isn't 5 minutes before they walk down the aisle. But if you let them know throughout that you're there for them no matter what, no questions asked, they'll get the picture.