A spread rather than a slice sits at the center, and that single fact decides how the whole thing behaves. Rillettes are pork (or sometimes duck or rabbit) cooked slowly in its own fat until the meat shreds into soft strands and emulsifies with that fat into a pale, coarse paste you spread with a knife rather than lay in shingles. The sandwich is the portable form of that paste: a crusted baguette, split, and a thick layer of rillettes worked into the crumb, usually with little else so the cured pork is what you taste.
The logic follows from the texture and the fat. Rillettes carry their own richness through every gram, so the sandwich needs no butter and no melted cheese to feel substantial; the paste is the filling and the binder at once, pressing into the open crumb and holding the loaf together as one mass. That richness also sets the constraint. It is salty and dense enough that the build stays lean and the counterweight stays sharp, a few cornichons or a stripe of strong mustard cutting the fat the way they do on a charcuterie board. The bread has to have a real crust, because a soft loaf collapses under a heavy, yielding spread that offers no structure of its own. It eats cool and best at room temperature, where the fat is soft and carries; chilled hard, the rillettes turn waxy and the loaf goes leaden.
Variations stay on the charcuterie shelf rather than wandering off it. A duck version is darker and gamier than the standard pork; a rabbit one is leaner and finer in the grain; the plainest is rillettes and bread alone, the spread standing as the whole sandwich. Each holds the potted, shredded paste as the fixed point and changes only what it is made from or what sharpens it. The Sandwich aux Rillettes belongs with the cured-meat sandwiches the catalog groups under Sandwich Saucisson & Charcuterie, the tradition that runs across France's regional curing shelves. Its specific contribution is a fat-bound, shreddable spread that behaves like its own condiment, so the sandwich's job is to carry it and stay out of its way.