Start with why not
In every creative brief, there is (or should be) a box on “why would they do it?”, or “why should they believe us?”. But i find another question equally helpful before you get into that.
“Why don't they already do it?”
This is how you get to a clear barrier (AKA human problem) that your work can help solve it. And chances are, unless you're brand new to a market, the answer isn't ‘lack of awareness’.
So your first job is not to prove why something works, but to work out why it's not working already. Strategy as less of an exercise of corroboration, and more an exercise in falsifiability.
This leaves you with three useful sets of questions to get started on any project:
“What do we want them to do?” gives you what the work is meant to communicate
“Why don't they do it?” gives you the barriers you need for the comms to feel relevant
“Why would they do it?” gives you the proof you need for the comms to feel truthful
Then, and only then, you should start thinking about how to make the work feel unforgettable.