The rule that decides it: can the filling survive the oven?
Heat-stable fillings bake INSIDE. A baton of dark chocolate (that's pain au chocolat), almond frangipane, cheese, cooked savoury fillings - place a modest amount at the wide end before rolling, keep it clear of the edges, roll snug. They warm, melt, set - all fine at 180°C.
Delicate creams go in AFTER. Pastry cream, whipped cream, mousses, ganache-and-cream blends curdle, split or boil out in the oven. Bake the croissant plain, cool it, then pipe through the ends or a small slit underneath.
Two cautions for the bake-inside route: less filling than you think (overfilled croissants burst at the seam and leak), and cold filling into cold dough - a warm filling melts the lamination from inside before the oven even starts. Almond croissants are conveniently both methods in one pastry: frangipane baked in a SPLIT baked croissant, twice-baked. Good gateway experiment.
