If you use grocery store sausage, the sausage casings on the Italian sausage will dissolve in the hot skillet; however, if you buy your meat from a butcher, that sausage casing is different than packaged sausage from the grocery store.  If you use "butcher-made" sausage, boil the links about 5-minutes before cooking on the stovetop to dissolve the casing, then transfer links to a large skillet and brown.