In sum: you create a charter document when you first have the idea for something. It’s a very simple document that you fill out and hand to the person who authorizes the time/resources you need for your project. I never use these documents instead of talking to people though; I talk to everyone before I write the document, and also when I pass it off. The document is like a cheatsheet about the idea, the paper version of the conversation you’re having. But a charter document is open and simple enough that you aren’t locking yourself into any one solution yet. It’s just saying: hey, here’s a problem I want to address. If you’re me and incapable of keeping any bright ideas to yourself, you are probably saying way more about what you want to do with this project in person than you are in paper. But at least you’re all acknowledging that yes, you’re going to spend some time thinking about this thing, and we all agree that it’s a good idea.
If you’re concerned about being called on your use of your time, you can modify this document to include a signature area. You can get someone to physically sign off on you investigating a project. At the very very start, this is the document they would be signing off on.
A charter document is a one-off. It just gets you started on the project. The end project might look very very different than the charter, but that’s okay. We can address any changes in the project in future documents, like the scope and alternatives. But more on that later.