I was tasked with making a dress from one of the collections I designed. The illustration and technical flat below indicate how this dress would look in its three-dimensional form. The dress called for a fabric insert at the neckline and an overlay at the waist.
The neckline insert was an interesting endeavor - I worked with a fabric that would drape easily and would not hold the shape intended. Thus, I reinforced it by interfacing it (ironing an interfacing on the back). I sewed the insert and then attached it to the fabric used in the remainder of the dress.
On the other hand, the drapiness of the same fabric was lovely in the overlay at the waist. There I was going for an easy drape that would not add bulk to the side of the dress. No interfacing was needed here.