The brief is what you want. The conversation is what you need.
Briefs are documents. They describe a desired outcome in terms the client can defend to their stakeholders. They are not, in our experience, reliable descriptions of the actual job.
We ask for a 30-minute call instead. In that call we're listening for the problem behind the problem. The site that needs to be rebuilt is usually covering for a workflow that doesn't exist, a team that hasn't been hired, or a product that hasn't been defined.
This is not a knock on clients. It's the nature of the work. You hire a studio because you don't know how to get from where you are to where you want to be. The brief is your best guess. The conversation is where we find the truth.
We send a proposal after that call. The proposal describes what we heard, what we think the actual job is, and what it would cost to do it. Sometimes the client says: that's not what we asked for. Usually they say: that's exactly what we need.