As a didactic example it was recently shown to me how horse-hair interfacing is properly fixated onto the under-collar, using a fascinating, yet strenuous, pattern of minuscule stitches attaching the inter-facing whilst forming the collar around your hand as you move, allowing you to pull both bias-cut layers simultaneously into a round fold, bestowing the rigid collar with an elegant roll-over.
Though I’m generally keen to use fusible interfacings, I’m thankful to my dear mentor for explaining me how natural interfacings are still more profitable in constructions like these.