In my case (Lisa), not only am I bigger than average, I'm also much squishier. This makes it difficult to know if the pinned corset is actually correct. In fact pinning the corset on me is quite a chore. Robin is a bit easier, but it's still not easy. Aside from basic fit, there's also a lot of decisions around what kind of fabric to use, and how many layers. We each made a muslin for fitting. Muslin stretches. Once we each got a muslin that fit, after at least three iterations, we needed to use the muslin as a pattern to make sure that we accounted for stretching. We also wanted to keep the layers to a minimum. We expect to wear these costumes in very warm weather. We ended up with coutil and linen. It feels amazing. The picture at top is Lisa's corset, with both layers cut and the first bone in it's channel. That's what we wanted to accomplish, and we did.
Here are a couple fitting pictures to illustrate the challenges of fitting: