I have recently been having a discussion with a colleague trying to get them to understand that, in an agile environment you don’t need to define the deliverables with finite detail but instead have a vision of what you need to achieve and work towards it in measurable chunks.
In order to do so I used the metaphor of someone who, looking into the distance can see their immediate destination close up, they can see the fences that bar their way, or shortcuts that will save time and effort and can plan their route in detail.
However as they look further into the distance and towards their eventual destination, the detail to which they can plan their route gets less and less. It’s only as they travel their route that they get to see the new barriers, new paths to tread and that they can plan the route for the the next phase in detail.
Have a view of the long term goals of your project but you can’t plan the whole thing in detail as things change was you go, plan the next phase in detail using your experience to make informed decisions, defining the detailed requirements for the each phase in turn refining your understanding of the project deliverables as you go.
And remember, the methodology is a tool to achieve the end goal not the goal itself.