All eyes will be on the bride, so as long as you conduct yourself like a lady, you will be fine. I attended my secretary's wedding years ago and was absolutely appalled when one of her bridesmaids dropped to the floor and did "The Alligator" dance. It would have been at least a bit better had she not chosen to wear a thong.
Not at all. But try and be as accommodating and helpful to the bride as possible. That's what bridesmaids are supposed to do. Ask her if she needs help, errands run or whatever. Help make her day special.