As always, nothing in the world can take the place of prudential judgement. When proposals should be written up to avoid senseless action and short-term thinking, and when they should be avoided because they would constitute bikeshedding or overthinking or whatever, is a matter of prudence and competence gained through experience. There is no algorithm that will allow you to mindlessly come to the correct decision.

A demo is no substitute for a written proposal and vice versa. The first is empirical, the latter is analytical and deliberative. Each has its proper place. Really, the demo is a way to corroborate the proposal, so it's a question of how rigorous the proposal needs to be. Some minor tweak might only require a quick discussion while a deep change will require deeper consideration.