A: Traditionally, once an attendant accepted the role, only illness or a death in the family was a reason to back out. Today, a critical business trip or work-related issue can force a groomsman or bridesmaid to have to step down. If this happens early in your planning, before all of the attendants have confirmed, you may ask someone else to fill the role—a close friend or relative whom you think would be honored by the invitation. It's a kind gesture to absorb any costs of his or her participation, as the substitute wouldn't have been expecting such a potential blow to the budget.
If an attendant cancels right before the event, then carry on without replacing them. Usually, issues involving the processional or table seating can be easily rearranged. Avoid asking someone else to stand in at the last minute—they will feel very second best. Know, too, that it's okay to have an uneven ratio of bridesmaids to groomsmen; one lucky guy or girl will just get to walk the recessional with two people on their arm. —Anna Post, The Emily Post Institute