This is much essential because each member of the team who is developing your product have their own perception and they see the picture standing inside the frame so missing even a single detail can lead to something very different from the expected. Functional Specifications Surely helps here.

I agree with you. The storyboards that you describe is another form of a functional spec. Whether the functional spec takes the form of a word document, a storyboard, a powerpoint presentation, does not matter – my point is that such a document with details on the features called out in the PRD is absolutely needed – just a PRD will not do.

I see the goal overall is to communicate a certain level of information to developers, testers, and others to ensure everyone is on the same page as far as what you’re developing. A functional spec is one way to do that, detailed stories with appropriate acceptance criteria (Agile PMs know what this means, others might not) is another, and a high fidelity prototype is yet another.

Personally, I’ve seen the combination of a prototype and stories be the best way to achieve the above goal, but other methods might work better for other teams, organizations, and types of products. Product managers ultimately are responsible for delivering the product, not the documentation — the documentation is just a means to an end, so PMs should use whatever means can best achieve that end.

Jeff
My blog: How To Be a Good Product Manager