Why a conversation outperforms a document
A proposal is prepared, reviewed and polished, which is exactly what makes it a poor discriminator. Every competent provider produces a competent document. A conversation is unrehearsed, and the quality of an answer given without preparation is a much better guide to whether a provider has thought about the work or is applying a familiar formula to another enquiry to a familiar enquiry they have answered many times before.
It also lets you follow up. A document cannot be asked what it means, but a person can be asked to be more specific, and the second answer is usually more revealing than the first. A provider who becomes more precise under gentle questioning is describing something real. One who becomes more general is describing something that was never particularly specific to begin with and will not become more specific once the work is won.
