This is the most important moment before the run-time, because at this point the larpmaker and the player enter a kind of contract or agreement. That contract had formal and informal, explicit and implicit parts. Usually it is mostly informal and implicit – which means players will fill in the rest with guesses and expectations. If you are organising for players with different larp backgrounds, this is a recipe for disaster. And if you are organising for your ordinary crowd, this gets in the way of clearly communicating innovations.
When your players are assuming stuff about your larp, two things inevitably happen: some people who will not enjoy or understand your work will sign up, to the detriment of all participants, and you will have to put in a tonne of work answering questions, clearing up misunderstandings, and monitoring online conversations to find out how your information is received. (Actually, you may have to do the last part anyway).
I think this post is the kind of thing we all feel that we ourselves know, but no other larp organiser is doing well enough. That is probably a clue that we are also not doing it well enough. Feel free to spread it around – and if I've forgotten something vital, do let me know in the comments so I can make additions! It'd be cool if this could work as a checklist when it's finished.