What Interfacing Does
Interfacing is a material bonded or stitched to the wrong side of fabric to add body, stability, and shape. It's invisible in the finished garment but responsible for why collars stand up, waistbands don't roll, and structured bodices hold their shape through a full convention day. Choosing the wrong interfacing (too stiff, too soft, wrong application method) creates visible problems that are difficult to correct after the garment is assembled.
Types of Interfacing
Fusible interfacing has a heat-activated adhesive on one side and is applied with an iron. It's the most common choice for most applications — quick to apply, available in multiple weights, and reliable when the application instructions are followed precisely. Sew-in interfacing is attached by stitching; better for fabrics that can't tolerate heat or for applications where the interfacing needs to be removable.
Boning for Structure
Boning creates rigid structure in bodices, corsets, and structured costume pieces. Spiral steel boning is flexible in all directions — appropriate for curved seams and anywhere the wearer needs to bend. Flat steel boning is rigid in one plane — used at centre front, centre back, and side seams where you want to prevent forward bending. Plastic boning is cheaper and easier to work with but significantly less effective at maintaining structure through a long wear.
Practical Application
For cosplay bodices: interface all bodice pieces before assembly; add boning in channels along the seam lines after the outer fabric and lining are joined; ensure boning ends are capped to prevent wear-through. For structured collars: a layer of organdy or crinoline between collar and facing creates stand without stiffness. When in doubt, make a mockup with the interfacing and test how it feels and looks before applying to fashion fabric.