He finds that many singers try to cover up shortcomings with bravado. It's a person who has on a beautiful gown, but underneath the undergarments may be all tattered and torn. . . . People may have a good instrument but not know how to use it, or they may sing beyond or below their capacity. If the singer is a possibility, the rest of the audition proves whether she has good technique and can go the distance. The next consideration is the singer's look.
People now expect singers to look the part.
Ideally, the duke should be handsome. Gilda is a young woman, so she should be trim and pretty. Costumers can work wonders, but there are some hard facts that can't be overcome with needle and thread. The man has to be a bodybuilder or she has to be lithe enough to do that. Otherwise, he'll have to drag her across the stage.