Going through an issue with someone involving a co-authored project. In hindsight, I've realized a written agreement needs to be both what the authors are agreeing to and what they are not agreeing to, what is allowed and what isn't allowed for the partners. Someone took a small permission and drove a semi through it.
Another author contacted me about what she could do with a project without the permission of the director who brought her in to it, and the director wasn't giving permission. Not much.
Really, all agreements among writers should be put into writing, even if it's just an email, including what will happen if the partners can't agree. Anyone who tells you a project should be done on trust is going to rape, loot, and pillage you, and then tell you that you knew what they were going to do when you went along without getting an agreement in words.