You should at least have talked about all the points that go into an incorporation document and some that don't (equity shares, vesting, time & work commitments, decision-making, roles, etc.) and have a verbal agreement that everyone's on-board with. You don't have to actually file the paperwork, which is expensive and creates ongoing administrative work. You should file the paperwork ASAP, but if a founder starts working on the project and then decides that his true desires lie elsewhere or he's not happy with the working arrangement, that's often easier to fix before there's a real company than afterwards.