The wedding was just a blur to Jean. He was scared that someone would find out who was in that ultra-feminine ensemble and also that he would mess up her part and spoil the whole wedding.
Jeff didn't care about this sort of thing as a general rule. How come it made such a difference when he was directly involved, such as being a bridesmaid? Did the wedding clothes change his way of thinking? Would wearing fancy dresses and lingerie make him a fairy or a queer? With Jill's coaching on the way to the church, Jeff was able to do Jane's part properly.
The most embarrassing moment was when the matron of honor walked up to Jean and reached inside the open front of her dress. She was rying to pin on a corsage. Jean was positive that she would notice that the bra was stuffed with pantyhose and discover the masquerade. She was ready to run from the church to escape, if she was exposed as an impostor. Luckily no one seemed to notice that it was Jeff posing as Jane