Rule of thumb in RPGs is to always save at dialogue branches related to story events, usually pivotal ones like killing or allying major characters. That means you might be saving every couple of minutes during heavy-plot moments. Then you save when all the dialogue is done and go back and test the momentous decisions. It sucks to do but that's why achievements are bull*****they guilt-trip you for not wasting your time or ruining your play experience with tedium. They're just a devious device to keep players invested in the product, taking advantage of our human psychology to pressure completion. That's why I ignore them.